Large reserves in allocating_section can cause stalls. We already log
reserve increase, but we don't know which table it belongs to:
lsa - LSA allocation failure, increasing reserve in section 0x600009f94590 to 128 segments;
Allocating sections used for updating row cache on memtable flush are
notoriously problematic. Each table has its own row_cache, so its own
allocating_section(s). If we attached table name to those sections, we
could identify which table is causing problems. In some issues we
suspected system.raft, but we can't be sure.
This patch allows naming allocating_sections for the purpose of
identifying them in such log messages. I use abstract_formatter for
this purpose to avoid the cost of formatting strings on the hot path
(e.g. index_reader). And also to avoid duplicating strings which are
already stored elsewhere.
Fixes#25799Closesscylladb/scylladb#27470
Partitions.db internally uses a piece of the partition key murmur
hash (the same hash which is used to compute the token and the
relevant bits in the bloom filter). Before this patch,
the Partitions.db reader computes the hash internally from the
`sstables::partition_key`.
That's a waste, because this hash is usually also computed
for bloom filter purposes just before that.
So in this patch we let the caller pass that hash instead.
The old index interface, without the hash, is kept for convenience.
In this patch we only add a new interface, we don't switch the callers
to it yet. That will happen in the next commit.
In later parts of the series, we add a second implementation
of `abstract_index_reader`. To do that, we want a header with
`abstract_index_reader`, but we don't need to pull in everything else
from `index_reader.hh`. So let's extract `abstract_index_reader`
to its own header.
`advance_context()` needs an ability to advance the index to
the partition immediately following the reader's current partition.
For this, it uses `abstract_index_reader::advance_to(dht::ring_position_view)`
But BTI (and any index format which stores only the prefixes of keys
instead of whole keys) can't implement `advance_to` with its current
semantics. The Data position returned by the index for a generic
`advance_to` might be off by one partition.
E.g. if the index stores prefixes `a`, `b`, `c`,
the index has no way to know if the first entry after `bb`
is `b` (which might correspond to `ba` as well as `bc`), or `c`.
However, BTI can be used exactly if the partition is known to
be present in the sstable. (In the above example, if `bb` is known
to be present in the sstable, then it must correspond to `b`.
So the index can reliably advance to `bb` or the first partition after it).
And this is enough for `advance_context()`, because the
current partition is known to be present.
So we can replace the usage of `advance_to` with an equivalent API call
which only works with present keys, but in exchange is implementable
by BTI.
This makes `advance_to` unused, so we remove it.
`advance_to_next_partition()` needs an ability to advance the index to
the partition immediately following the reader's current partition.
For this, it uses `abstract_index_reader::advance_to(dht::ring_position_view)`
But BTI (and any index format which stores only the prefixes of keys
instead of whole keys) can't implement `advance_to` with its current
semantics. The Data position returned by the index for a generic
`advance_to` might be off by one partition.
E.g. if the index stores prefixes `a`, `b`, `c`,
the index has no way to know if the first entry after `bb`
is `b` (which might correspond to `ba` as well as `bc`), or `c`.
However, BTI can be used exactly if the partition is known to
be present in the sstable. (In the above example, if `bb` is known
to be present in the sstable, then it must correspond to `b`.
So the index can reliably advance to `bb` or the first partition after it).
And this is enough for `advance_to_next_partition()`, because the
current partition is known to be present.
So we can replace the usage of `advance_to` with an equivalent API call
which only works with present keys, but in exchange is implementable
by BTI.
BTI indexes only store encoded prefixes of partition keys,
not the whole keys. They can't reliably implement `get_partition_key`.
The index reader interface must be weakened and callers must
be adapted.
This is a refactoring patch in preparation for BTI indexes. It contains no functional changes (or at least it's not intended to).
In this patch, we modify the sstable readers to use index readers through a new virtual `abstract_index_readers` interface.
Later, we will add BTI indexes which will also implement this interface.
This interface contains the methods of `index_reader` which are needed by sstable readers, and leaves out all other methods, such as `current_clustered_cursor`.
Not all methods of this interface will be implementable by a trie-based index later. For example, a trie-based index can't provide a reliable `get_partition_key()`, because — unlike the current index — it only stores partition keys for partitions which have a row index. So the interface will have to be further restricted later. We don't do that in this patch because that will require changes to sstable reader logic, and this patch is supposed to only include cosmetic changes.
No backports needed, this is a preparation for new functionality.
Closesscylladb/scylladb#25000
* github.com:scylladb/scylladb:
sstables: add sstable::make_index_reader() and use where appropriate
sstables/mx: in readers, use abstract_index_reader instead of index_reader
sstables: in validate(), use abstract_index_reader instead of index_reader where possible
test/lib/index_reader_assertions: accept abstract_index_reader instead of index_reader
sstables/index_reader: introduce abstract_index_reader
sstables/index_reader: extract a prefetch_lower_bound() method
We want to implement BTI indexes in Scylla.
After we do that, some sstables will use a BTI index reader,
while others will use the old BIG index reader.
To handle that, we can expose a common virtual "index reader"
interface to sstable readers. This is what this patch does.
This interface can't be quite fully implemented by a BTI index,
because some methods returns keys which a BIG index stores,
but a BTI index doesn't. So it will be further restricted in future
patches. But for now, we only extract *all* methods currently
used by the readers to a virtual interface.
The sstable reader reaches directly for a `clustered_index_cursor`.
But a BTI index reader won't be able to implement
`clustered_index_cursor`, because a BTI index doesn't store
full clustering keys, only some trie-encoded prefixes.
So we want to weaken the dependency. Instead of reaching
for `clustered_index_cursor`, we add a method which expresses
our intent, and we let `index_reader` touch the cursor internally.
Convert all necessary methods to be awaitable. Start using `make_data_or_index_source`
when creating data_source for data and index components.
For proper working of compressed/checksummed input streams, start passing
stream creator functors to `make_(checksummed/compressed)_file_(k_l/m)_format_input_stream`.
Refactor readers and sources to support coroutine usage in
preparation for integration with `make_data_or_index_source`.
Move coroutine-based member initialization out of constructors
where applicable, and defer initialization until first use.
So parse errors on corrupt SSTables don't result in crashes, instead
just aborting the read in process.
There are a lot of SCYLLA_ASSERT() usages remaining in sstables/. This
patch tried to focus on those usages which are in the read path. Some
places not only used on the read path may have been converted too, where
the usage of said method is not clear.
these unused includes were identifier by clang-include-cleaner. after
auditing these source files, all of the reports have been confirmed.
please note, because quite a few source files relied on
`utils/to_string.hh` to pull in the specialization of
`fmt::formatter<std::optional<T>>`, after removing
`#include <fmt/std.h>` from `utils/to_string.hh`, we have to
include `fmt/std.h` directly.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Reads which need sstable index were computing
column_values_fixed_lengths each time. This showed up in perf profile
for a sstable-read heavy workload, and amounted to about 1-2% of time.
Computing it involves type name parsing.
Avoid by using cached per-sstable mapping. There is already
sstable::_column_translation which can be used for this. It caches the
mapping for the least-recently used schema. Since the cursor uses the
mapping only for primary key columns, which are stable, any schema
will do, so we can use the last _column_translation. We only need to
make sure that it's always armed, so sstable loading is augmented with
arming with sstable's schema.
Also, fixes a potential use-after-free on schema in column_translation.
Closesscylladb/scylladb#21347
* github.com:scylladb/scylladb:
sstables: Fix potential use-after-free on column_translation::column_info::name
sstables: Avoid computing column_values_fixed_lengths on each read
since fedora 38 is EOL. and fedora 39 comes with fmt v10.0.0, also,
we've switched to the build image based on fedora 40, which ships
fmt-devel v10.2.1, there is no need to support fmt < 10.
in this change, we drop the support fmt < 10.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#21847
Reads which need clustering index cursor were computing
column_values_fixed_lengths each time. This showed up in perf profile
for a sstable-read heavy workload, and amounted to about 1%.
Avoid by using cached per-sstable mapping. There is already
sstable::_column_translation which can be used for this. It caches the
mapping for the most recently used schema. Since the cursor uses the
mapping only for primary key columns, which are stable, any schema
will do, so we can use the last _column_translation. We only need to
make sure that it's always armed, so sstable loading is augmented with
arming with sstable's schema.
Terminology note: in the context of this series, "index page" means an contiguous segment of the index file starting (inclusive) at a key corresponding to a summary entry and ending (exclusive) before the key corresponding to the next summary entry. "Index pages" are not related to filesystem pages.
---
In a single-partition read, if the searched partition key is the first key in its index page, we start scanning the index for that key starting at the previous index page (inclusive), even though we could start directly from the key's page. Similarly, if the searched partition key is absent from the sstable and lies after all other keys in its appropriate page, we additionally scan the next page, even though it's known from the summary that it can't possibly contain the key.
Those cases are wasteful. It's worse than it might seem at first glance. When partitions are small, only a small fraction of search keys fulfills those conditions (i.e. "first key in its page" or "an absent key greater than the last key in its page"), so the waste doesn't matter much. But when partitions are big enough, every index page contains only one partition key (and a promoted index for that partition), which directly means that *all* search keys fulfill the conditions, which means that total index reading work is two times bigger than what it should be.
In addition, there is a secondary performance bug which, when the aforementioned conditions are fulfilled, causes *additional* I/O to happen *past* the index reads which are actually parsed and used. In effect, the index I/O in single-partition reads might be not just doubled, but even tripled (that's for IOPS — throughput might be multiplied even more), all because of a slight inaccuracy in the edge cases.
This series fixes those inefficiencies by tightening the edge cases and ensuring that single-partition reads always read only a single index page.
Here's an example where we query the first row (i.e. `LIMIT 1`) of a certain partition key, in a table with large (1 MB) promoted indexes. Before the patch, the lookup of the lower bound involves 3 serialized disk reads (as described above) to subsequent index pages, and even the lookup of the upper bound involves 2 disk reads:
```
Execute CQL3 query
Parsing a statement [shard 0]
Processing a statement for authenticated user: anonymous [shard 0]
Executing read query (reversed false) [shard 0]
Creating read executor for token -1297921881139976049 with all: [127.11.11.1] targets: [127.11.11.1] repair decision: NONE [shard 0]
Creating never_speculating_read_executor - speculative retry is disabled or there are no extra replicas to speculate with [shard 0]
read_data: querying locally [shard 0]
Start querying singular range {{-1297921881139976049, pk{00023130}}} [shard 0]
[reader concurrency semaphore user] admitted immediately [shard 0]
[reader concurrency semaphore user] executing read [shard 0]
Reading key {-1297921881139976049, pk{00023130}} from sstable ./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Data.db [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 38359040 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 38391808 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 38359040, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 38391808, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 39370752 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 39403520 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 39370752, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 39403520, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 40378368 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 40411136 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 40378368, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 40411136, successfully read 32768 bytes [shard 0]
upper_bound_cache_only({position: clustered, ckp{}, 1}): no upper bound [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 40378368 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 40411136 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 40378368, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 40411136, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 41390080 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 41422848 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 41390080, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 41422848, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Data.db: scheduling bulk DMA read of size 21926 at offset 819200 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Data.db: finished bulk DMA read of size 21926 at offset 819200, successfully read 24576 bytes [shard 0]
Page stats: 1 partition(s), 0 static row(s) (0 live, 0 dead), 1 clustering row(s) (1 live, 0 dead), 0 range tombstone(s) and 0 cell(s) (0 live, 0 dead) [shard 0]
Querying is done [shard 0]
Done processing - preparing a result [shard 0]
Request complete
```
After the patch, the lookup of each bound involves 1 read:
```
Execute CQL3 query
Parsing a statement [shard 0]
Processing a statement for authenticated user: anonymous [shard 0]
Executing read query (reversed false) [shard 0]
Creating read executor for token -1297921881139976049 with all: [127.11.11.1] targets: [127.11.11.1] repair decision: NONE [shard 0]
Creating never_speculating_read_executor - speculative retry is disabled or there are no extra replicas to speculate with [shard 0]
read_data: querying locally [shard 0]
Start querying singular range {{-1297921881139976049, pk{00023130}}} [shard 0]
[reader concurrency semaphore user] admitted immediately [shard 0]
[reader concurrency semaphore user] executing read [shard 0]
Reading key {-1297921881139976049, pk{00023130}} from sstable ./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Data.db [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 39370752 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 39403520 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 39370752, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 39403520, successfully read 32768 bytes [shard 0]
upper_bound_cache_only({position: clustered, ckp{}, 1}): no upper bound [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 40378368 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: scheduling bulk DMA read of size 32768 at offset 40411136 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 40378368, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Index.db: finished bulk DMA read of size 32768 at offset 40411136, successfully read 32768 bytes [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Data.db: scheduling bulk DMA read of size 21926 at offset 819200 [shard 0]
./workdir_01/data/ks/t-536c31f09a9c11efbd5082a6aa3e8d0c/me-3gky_0v18_3rgjk2dsjae431s4uz-big-Data.db: finished bulk DMA read of size 21926 at offset 819200, successfully read 24576 bytes [shard 0]
Page stats: 1 partition(s), 0 static row(s) (0 live, 0 dead), 1 clustering row(s) (1 live, 0 dead), 0 range tombstone(s) and 0 cell(s) (0 live, 0 dead) [shard 0]
Querying is done [shard 0]
Done processing - preparing a result [shard 0]
Request complete
```
Doesn't have to be backported, since the problem only affects performance, not correctness, and it has been present since forever.
Closesscylladb/scylladb#20897
* github.com:scylladb/scylladb:
index_reader: remove a piece of misguided code involved in single-partition reads
index_reader: in single-partition reads, don't read more than one page
index_reader: fix unnecessary reads of preceding index pages
This is optimization.
Example:
block0: start=aaa, end=aaA
block1: start=bbb, end=bbB
block2: whatever
Before the patch, advance_to("aAA") would skip to block0, and upper
bound probe would skip to block1. This way, the reader would read the
range of block0 from the data file.
After the patch, "end" position is taken into account, so
advance_to("aAA") will notice that block0 doesn't contain the position
and will skip to block1. This is especially important for dense
indexes, as it allows us to skip accessing data file if the search key
is missing.
It also solves the edge case problem related to the fact that single
row reads are using a range which with positions which are not equal
to the key, but are before(key) and after(key) for the lower bound and
upper bound respectively. Before the patch, advance_to(before("bbb"))
would skip to block0, before the position is before the block1's
start. And upper bound probe for after("bbb") would point to
block2. This way the read would scan block0 needlessly. After the
patch, advance_to(before("bbb")) will skip to block1 because we notice
based on "end" that block0 doesn't contain the position.
This change also ensures that the start position of the upper bound
entry of the after_key(pos), where pos is the last advance_to()
position, is warm in cache. This is needed to optimize single-row
reads with a dense index so that they always read exactly one promoted
index block. For this to work, probe_upper_bound() for the
after_key(row) always needs to find the upper bound block in
cache.
Single-row reads from large partition issue 64 KiB reads to the data file,
which is equal to the default span of the promoted index block in the data file.
If users would want to reduce selectivity of the index to speed up single-row reads,
this won't be effective. The reason is that the reader uses promoted index
to look up the start position in the data file of the read, but end position
will in practice extend to the next partition, and amount of I/O will be
determined by the underlying file input stream implementation and its
read-ahead heuristics. By default, that results in at least 2 IOs 32KB each.
There is already infrastructure to lookup end position based on upper
bound of the read, but it's not effective becasue it's a
non-populating lookup and the upper bound cursor has its own private
cached_promoted_index, which is cold when positions are computed. It's
non-populating on purpose, to avoid extra index file IO to read upper
bound. In case upper bound is far-enough from the lower bound, this
will only increase the cost of the read.
The solution employed here is to warm up the lower bound cursor's
cache before positions are computed, and use that cursor for
non-populating lookup of the upper bound.
We use the lower bound cursor and the slice's lower bound so that we
read the same blocks as later lower-bound slicing would, so that we
don't incur extra IO for cases where looking up upper bound is not
worth it, that is when upper bound is far from the lower bound. If
upper bound is near lower bound, then warming up using lower bound
will populate cached_promoted_index with blocks which will allow us to
locate the upper bound block accurately. This is especially important
for single-row reads, where the bounds are around the same key. In
this case we want to read the data file range which belongs to a
single promoted index block. It doesn't matter that the upper bound
is not exactly the same. They both will likely lie in the same block,
and if not, binary search will bring adjacent blocks into cache. Even
if upper bound is not near, the binary search will populate the cache
with blocks which can be used to narrow down the data file range
somewhat.
Fixes#10030.
The change was tested with perf-fast-forward.
I populated the data set with `column_index_size_in_kb` set to 1
scylla perf-fast-forward --populate --run-tests=large-partition-slicing --column-index-size-in-kb=1
Test run:
build/release/scylla perf-fast-forward --run-tests=large-partition-select-few-rows -c1 --keep-cache-across-test-cases --test-case-duration=0
This test reads two rows from the middle of a large partition (1M
rows), of subsequent keys. The first read will miss in the index file
page cache, the second read will hit.
Notice that before the change, the second read issued 2 aio requests worth of 64KiB in total.
After the change, the second read issued 1 aio worth of 2 KiB. That's because promoted index block is larger than 1 KiB.
I verified using logging that the data file range matches a single promoted index block.
Also, the first read which misses in cache is still faster after the change.
Before:
running: large-partition-select-few-rows on dataset large-part-ds1
Testing selecting few rows from a large partition:
stride rows time (s) iterations frags frag/s mad f/s max f/s min f/s avg aio aio (KiB) blocked dropped idx hit idx miss idx blk c hit c miss c blk allocs tasks insns/f cpu
500000 1 0.009802 1 1 102 0 102 102 21.0 21 196 2 1 0 1 1 0 0 0 568 269 4716050 53.4%
500001 1 0.000321 1 1 3113 0 3113 3113 2.0 2 64 1 0 1 0 0 0 0 0 116 26 555110 45.0%
After:
running: large-partition-select-few-rows on dataset large-part-ds1
Testing selecting few rows from a large partition:
stride rows time (s) iterations frags frag/s mad f/s max f/s min f/s avg aio aio (KiB) blocked dropped idx hit idx miss idx blk c hit c miss c blk allocs tasks insns/f cpu
500000 1 0.009609 1 1 104 0 104 104 20.0 20 137 2 1 0 1 1 0 0 0 561 268 4633407 43.1%
500001 1 0.000217 1 1 4602 0 4602 4602 1.0 1 2 1 0 1 0 0 0 0 0 110 26 313882 64.1%
(cherry picked from commit dfb339376aff1ed961b26c4759b1604f7df35e54)
This patch removes a piece of code which, according to the comment,
allows for forwarding the index reader even if it was created as a
single-partition reader.
For single-partition reads, the input_stream
used by the reader is limited to the single index page containing the partition,
since reading the index file past that point would be a waste.
Because of this limit, such an index reader can't be forwarded/advanced.
The dubious piece of code gets around that by unsetting the stream and ensuring
it will be re-created, this time without the limit, if the index is advanced.
But there is no use for this. The idea of a "single-partition reader"
exist as an optimization. It's illegal to forward single-partition readers,
and it doesn't make sense to attempt that. (If there's a need for forwarding,
just don't create a single-partition reader).
I suspect this piece of code was written due to a misunderstanding.
Before the previous patch in this series, when the searched partition key
was the first key in its page, the index reader would scan the preceding
page first, realize it made a mistake, and advance to the next, correct page.
I suspect this piece of code was written to make this work.
But this is, in fact, undesirable. The fact that the index reader was working
like this was a performance bug. In the single-partition case there's
never an inherent reason to start with the wrong page. The index logic can be
corrected to always start with the right page, and that's what the previous
patch in this series does. And with that, there is no need to support advancing
anymore, and the dubious piece of code can be erased.
We also add an assert to emphasize that advancing a single-partition reader is
illegal.
When looking for a partition key in the index, we scan the index from the
first index page which can possibly contain the key.
In a single-partition read, there is never a reason to read beyond that
page. After the previous patch in this series, it's guaranteed that
the first key in the next page is strictly greater than the searched key.
So if the searched key is greater than the last key in the first page,
then it is neither in the first nor the second page -- it must be absent
from the sstable.
But with the current logic, we read the second index page anyway, and
the realization that the key is absent happens higher in the call chain.
This patch optimizes that inefficiency by immediately returning EOF
if a single-partition read doesn't find the key in the first page.
Returning "end of file" even though we didn't actually go beyond the
end of file is hacky, but I don't see any other non-invasive way
of communicating to the caller that the partition is absent.
Some caller of the index could possibly assume that returning EOF
proves that the searched key is greater than all keys in the sstable.
I don't think any such caller exists today, but it's a possible
place for confusion.
Together with the previous patch in this series, this patch guarantees
that a single-partition read only accesses a single index page.
This fixes a weird secondary performance bug.
Due to some misunderstanding in the logic, when during a single-partition read
we scan two index pages, the second index page is scanned via an input_stream
created without an upper I/O limit, which means that we additionally read
a full read-ahead (currently: 64 kiB) past the second index page for no reason
whatsoever.
After this and the previous patch, a single-partition read always reads exactly one
index page, so the above problem cannot occur.
When setting the index to position X, we first look for the first summary
entry N such that N >= X. Then we load the index page preceding N and
scan it for the first partition key P such that P >= X.
If there is no such key in this page, then we scan the next page
(starting with N) for such key. (In this case it's always the first key).
For example, assume we have:
summary: A C E
index: A B C D E F
If we look up "B" in the index, then we first locate summary entry "C",
then we scan the index for B, starting from "A". This is all fine.
But when we look for "C" in the index, then we do the exactly the same
-- we scan the index for "C" starting from "A". This is wasteful, because
we can start scanning from "C".
To avoid this inefficiency, we should be looking for N > X, not N >= X.
This patch fixes that.
In addition, this fixes a second, weirder performance bug.
Due to some misunderstanding in the logic, when during a single-partition read
we scan two index pages, the second index page is scanned via an input_stream
created without an upper I/O limit, which means that we additionally read
a full read-ahead (currently: 64 kiB) past the second index page for no reason
whatsoever. After this patch, a single-partition read always reads exactly one
index page, so the above problem cannot occur.
assert() is traditionally disabled in release builds, but not in
scylladb. This hasn't caused problems so far, but the latest abseil
release includes a commit [1] that causes a 1000 insn/op regression when
NDEBUG is not defined.
Clearly, we must move towards a build system where NDEBUG is defined in
release builds. But we can't just define it blindly without vetting
all the assert() calls, as some were written with the expectation that
they are enabled in release mode.
To solve the conundrum, change all assert() calls to a new SCYLLA_ASSERT()
macro in utils/assert.hh. This macro is always defined and is not conditional
on NDEBUG, so we can later (after vetting Seastar) enable NDEBUG in release
mode.
[1] 66ef711d68Closesscylladb/scylladb#20006
When an abort is requested, stop further reading of the index file and
throw and exception from index_consume_entry_context::process_state().
Signed-off-by: Lakshmi Narayanan Sreethar <lakshmi.sreethar@scylladb.com>
Store abort source inside sstables::index_consume_entry_context, so that
the next patch can implement cancelling the index read when abort is
requested.
Signed-off-by: Lakshmi Narayanan Sreethar <lakshmi.sreethar@scylladb.com>
managed_bytes is implemented as chain of blob_storage objects.
Each blob_storage contains 24 bytes of metadata. But in the most
common case -- when there is only a single element in the chain --
16 bytes of this metadata is trivial/unused.
This is regrettable waste because managed_bytes is used for every
database cell in the memtables and cache. It means that every value
of size >= 7 bytes (smaller ones fit in the inline storage of
managed_bytes) receives 16 bytes of useless overhead.
To correct that, this patch adds to managed_bytes an alternative storage
layout -- used for buffers small enough to fit in one contiguous
fragment -- which only stores the necessary minimum of metadata.
(That is: a pointer to the parent, to facilitate moving the storage during
memory defragmentation).
before this change, we rely on the default-generated fmt::formatter
created from operator<<, but fmt v10 dropped the default-generated
formatter.
in this change, in order to enable the code in the header to
access the formatter without being moved down after the full specialization's
definition, we
* move the enum definition out of the class and before the
class,
* rename the enum's name from state to index_consume_entry_context_state
* define a formatter for index_consume_entry_context_state
* remove its operator<<().
as fmt v10 is able to use `format_as()` as a fallback, the formatter
full specialization is guarded with `#if FMT_VERSION < 10'00'00`. we
will remove it after we start build with fmt v10.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#16204
Move partition_index_cache stats from a thread_local variable
to cache_tracker. After the change, partition_index_cache
receives a reference to the stats via constructor, instead of
referencing a global.
This is needed so that cache_tracker can know the memory usage
of index caches (for cache eviction purposes) without relying on
globals.
But it also makes sense even without that motive.
Move cached_file metrics from a thread_local variable
to cache_tracker.
This is needed so that cache_tracker can know
the memory usage of index caches (for purposes
of cache eviction) without relying on globals.
But it also makes sense even without that motive.
Prevent switch case statements from falling through without annotation
([[fallthrough]]) proving that this was intended.
Existing intended cases were annotated.
Closes#14607
In that level no io_priority_class-es exist. Instead, all the IO happens
in the context of current sched-group. File API no longer accepts prio
class argument (and makes io_intent arg mandatory to impls).
So the change consists of
- removing all usage of io_priority_class
- patching file_impl's inheritants to updated API
- priority manager goes away altogether
- IO bandwidth update is performed on respective sched group
- tune-up scylla-gdb.py io_queues command
The first change is huge and was made semi-autimatically by:
- grep io_priority_class | default_priority_class
- remove all calls, found methods' args and class' fields
Patching file_impl-s is smaller, but also mechanical:
- replace io_priority_class& argument with io_intent* one
- pass intent to lower file (if applicatble)
Dropping the priority manager is:
- git-rm .cc and .hh
- sed out all the #include-s
- fix configure.py and cmakefile
The scylla-gdb.py update is a bit hairry -- it needs to use task queues
list for IO classes names and shares, but to detect it should it checks
for the "commitlog" group is present.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
Closes#13963
Most of creators of index_reader construct it with default prio class,
null trace pointer and use_caching::yes. Assigning implicit defaults to
constructor arguments keeps the code shorter and easier to read.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
A "while at it" cleanup. When pathing the method (next patch) it turned
out that there are no other callers other than local class, so it _is_
private.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
these warnings are found by Clang-17 after removing
`-Wno-unused-lambda-capture` and '-Wno-unused-variable' from
the list of disabled warnings in `configure.py`.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
When closing _lower_bound and *_upper_bound
in the final close() call, they are currently left with
an engaged current_list member.
If the index_reader uses a _local_index_cache,
it is evicted with evict_gently which will, rightfully,
see the respective pages as referenced, and they won't be
evicted gently (only later when the index_reader is destroyed).
Reset index_bound.current_list on close(index_bound&)
to free up the reference.
Ref #12271
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Closes#12370
Due to an oversight, the local index cache isn't evicted gently
when _upper_bound existed. This is a source of reactor stalls.
Fix that.
Fixes#12271Closes#12364
Currently the sstable::filename(Index) is used in several places that
get the filename as a printable or throwable string and don't treat is
as a real location of any file.
For those, add the index_filename() helper symmetrical to toc_filename()
and (in some sense) the get_filename() one.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
sstable::get_filename() constructs the filename from components, which
takes some work. It happens to be called on every
index_reader::index_reader() call even though it's only used for TRACE
logs. That's 1700 instructions (~1% of a full query) wasted on every
SSTable read. Fix that.
Closes#11485
Commit e8f3d7dd13 added eof() checks to public partition-level
advance_to() methods, to ensure we do not attempt to re-read the last
page of the index when at eof(). It was noted however that this check
would be safer in advance_to(index_bound&, dht::ring_position_view)
because that is the method that all these higher-level methods end up
calling. Placing the check there would guarantee safety for all such
operations. This path does exactly that: it pushes down the check to
said method. One change needed for this to work is to check eof on the
bound that is currently advanced, instead of unconditionally checking
the lower bound.
Closes#10531
Attempting to call advance_to() on the index, after it is positioned at
EOF, can result in an assert failure, because the operation results in
an attempt to move backwards in the index-file (to read the last index
page, which was already read). This only happens if the index cache
entry belonging to the last index page is evicted, otherwise the advance
operation just looks-up said entry and returns it.
To prevent this, we add an early return conditioned on eof() to all the
partition-level advance-to methods.
A regression unit test reproducing the above described crash is also
added.
Due to fd62fba985
scoped enums are not automatically converted to integers anymore,
this is the intended behavior, according to the fmtlib devs.
A bit nicer solution would be to use `std::to_underlying`
instead of a direct `static_cast`, but it's not available until
C++23 and some compilers are still missing the support for it.
Tests: unit(dev)
Signed-off-by: Pavel Solodovnikov <pa.solodovnikov@scylladb.com>
All entries from a single partition can be found in a
single summary page.
Because of that, in cases when we know we want to read
only one partition, we can limit the underyling file
input_stream to the range of the page.
Signed-off-by: Wojciech Mitros <wojciech.mitros@scylladb.com>
Currently, when advancing one of index_reader's bounds,
we're creating a new index_consume_entry_context with a new
underlying file input_stream for each new page.
For either bound, the streams can be reused, because
the indexes of pages that we are reading are never
decreasing.
This patch adds a index_consume_entry_context to each of
index_reader's bounds, so that for each new page, the same
file input_stream is used.
As a result, when reading consecutive pages, the reads that
follow the first one can be satisfied by the input_stream's
read aheads, decreasing the number of blocking reads and
increasing the throughput of the index_reader.
Fixes#2388
Signed-off-by: Wojciech Mitros <wojciech.mitros@scylladb.com>
