When periodically reloading the values in the loading_cache, we would
iterate over the list of entries and call the load() function for
those which need to be reloaded.
For some concrete caches, load() can remove the entry from the LRU set,
and can be executed inline from the parallel_for_each(). This means we
could potentially keep iterating using an invalidated iterator.
Fix this by using a temporary container to hold those entries to be
reloaded.
Spotted when reading the code.
Also use if constexpr and fix the comment in the function containing
the changes.
Signed-off-by: Duarte Nunes <duarte@scylladb.com>
Message-Id: <20180712124143.13638-1-duarte@scylladb.com>
The continuation attached to _load() needs the key of the loaded entry
to check whether it was disposed during the load. However if _load()
invalidates the entry the continuation's capture line will access
invalid memory while trying to obtain the key.
To avoid this save a copy of the key before calling _load() and pass it
to both _load() and the continuation.
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <b571b73076ca863690f907fbd3fb4ff54e597b28.1531393608.git.bdenes@scylladb.com>
"
If there is a lot of partitions in the index page, index_list may grow large
and require large contiguous blocks of memory, because it's based on
std::vector. That puts pressure on the memory allocator, and if memory is
fragmented, may not be possible to satisfy without a lot of eviction. Switch
to chunked_vector to avoid this.
Refs #3597
"
* 'tgrabiec/avoid-large-alloc-in-index-reader' of github.com:tgrabiec/scylla:
sstables: Switch index_list to chunked_vector to avoid large allocations
utils: chunked_vector: Do not require T to be default-constructible for clear()
utils: chunked_vector: Implement front()
resize(), used by clear(), requires T to be default-constructible in
case the vector is expanded. It's not actually needed for clearing,
and there will be users which use clear() with
non-default-constructible T, so implement clear() without using
resize().
std::function's move constructor is not noexcept, so observer's move
constructor and assignment operator also cannot be. Switch to Seastar's
noncopyable_function which provides better guarantees.
Tests: observer_tests (release)
Message-Id: <20180710073628.30702-1-avi@scylladb.com>
An observable is used to decouple an information producer from a consumer
(in the same way as a callback), while allowing multiple consumers (called
observers) to coexist and to manage their lifetime separately.
Two classes are introduced:
observable: a producer class; when an observable is invoked all observers
receive the information
observer: a consumer class; receives information from a observable
Modelled after boost::signals2, with the following changes
- all signals return void; information is passed from the producer to
the consumer but not back
- thread-unsafe
- modern C++ without preprocessor hacks
- connection lifetime is always managed rather than leaked by default
- renamed to avoid the funky "slot" name
Message-Id: <20180709172726.5079-1-avi@scylladb.com>
"
Partition snapshots go away when the last read using the snapshot is done.
Currently we will synchronously attempt to merge partition versions on this event.
If partitions are large, that may stall the reactor for a significant amount of time,
depending on the size of newer versions. Cache update on memtable flush can
create especially large versions.
The solution implemented in this series is to allow merging to be preemptable,
and continue in the background. Background merging is done by the mutation_cleaner
associated with the container (memtable, cache). There is a single merging process
per mutation_cleaner. The merging worker runs in a separate scheduling group,
introduced here, called "mem_compaction".
When the last user of a snapshot goes away the snapshot is slided to the
oldest unreferenced version first so that the version is no longer reachable
from partition_entry::read(). The cleaner will then keep merging preceding
(newer) versions into it, until it merges a version which is referenced. The
merging is preemtable. If the initial merging is preempted, the snapshot is
enqueued into the cleaner, the worker woken up, and merging will continue
asynchronously.
When memtable is merged with cache, its cleaner is merged with cache cleaner,
so any outstanding background merges will be continued by the cache cleaner
without disruption.
This reduces scheduling latency spikes in tests/perf_row_cache_update
for the case of large partition with many rows. For -c1 -m1G I saw
them dropping from >23ms to 1-2ms. System-level benchmark using scylla-bench
shows a similar improvement.
"
* tag 'tgrabiec/merge-snapshots-gradually-v4' of github.com:tgrabiec/scylla:
tests: perf_row_cache_update: Test with an active reader surviving memtable flush
memtable, cache: Run mutation_cleaner worker in its own scheduling group
mutation_cleaner: Make merge() redirect old instance to the new one
mvcc: Use RAII to ensure that partition versions are merged
mvcc: Merge partition version versions gradually in the background
mutation_partition: Make merging preemtable
tests: mvcc: Use the standard maybe_merge_versions() to merge snapshots
Fixes#3546
Both older origin and scylla writes "known" compressor names (i.e. those
in origin namespace) unqualified (i.e. LZ4Compressor).
This behaviour was not preserved in the virtualization change. But
probably should be.
Message-Id: <20180627110930.1619-1-calle@scylladb.com>
"
Cache tracker is a thread-local global object that indirectly depends on
the lifetimes of other objects. In particular, a member of
cache_tracker: mutation_cleaner may extend the lifetime of a
mutation_partition until the cleaner is destroyed. The
mutation_partition itself depends on LSA migrators which are
thread-local objects. Since, there is no direct dependency between
LSA-migrators and cache_tracker it is not guarantee that the former
won't be destroyed before the latter. The easiest (barring some unit
tests that repeat the same code several billion times) solution is to
stop using globals.
This series also improves the part of LSA sanitiser that deals with
migrators.
Fixes#3526.
Tests: unit(release)
"
* tag 'deglobalise-cache-tracker/v1-rebased' of https://github.com/pdziepak/scylla:
mutation_cleaner: add disclaimer about mutation_partition lifetime
lsa: enhance sanitizer for migrators
lsa: formalise migrator id requirements
row_cache: deglobalise row cache tracker
Current LSA sanitizer performs only basic checks on the migrators use,
without doing any additonal reporting in case an error is detected. This
patch enhances it so that when a problem is detected relevant stack
traces get printed.
object_descriptor uses special encoding for migrator ids which assumes
that the valid ones are in a range smaller than uint32_t. Let's add some
static asserts that make this fact more visible.
This commit adds a helper class reusable_buffer which can be used to
avoid excessive memory allocations of large buffers when bytes_ostream
needs to be linearised. The idea is that reusable_buffer in most cases
is going to be thread local so that multiple continuation chains can
reuse the same large buffer.
Represents a deferring operation which defers cooperatively with the caller.
The operation is started and resumed by calling run(), which returns
with stop_iteration::no whenever the operation defers and is not
completed yet. When the operation is finally complete, run() returns
with stop_iteration::yes.
This allows the caller to:
1) execute some post-defer and pre-resume actions atomically
2) have control over when the operation is resumed and in which context,
in particular the caller can cancel the operation at deferring points.
It will be used to implement deferring partition_version::apply_to_incomplete().
This overload alows searching the elements by an arbitrary key as long as it is "hashable"
to the same values as the default key and if there is a comparator for
this new key.
Signed-off-by: Vlad Zolotarov <vladz@scylladb.com>
remove(key): removes the entry with the given key if exists, otherwise does nothing.
remote(iterator): removes an entry by a given iterator (returned from loading_cache::find()).
Signed-off-by: Vlad Zolotarov <vladz@scylladb.com>
"
Main optimization is in the patch titled "lsa: Reduce amount of segment compactions".
I measured 50% reduction of cache update run time in a steady state for an
append-only workload with large partition, in perf_row_cache_update version from:
c3f9e6ce1f/tests/perf_row_cache_update.cc
Other workloads, and other allocation sites probably also could see the
improvement.
"
* tag 'tgrabiec/reduce-lsa-segment-compactions-v1' of github.com:tgrabiec/scylla:
lsa: Expose counters for allocation and compaction throughput
lsa: Reduce amount of segment compactions
lsa: Avoid the call to segment_pool::descriptor() in compact()
lsa: Make reclamation on reserve refill more efficient
Reclaiming memory through segment compaction is expensive. For
occupancy of 85%, in order to reclaim one free segment, we need to
compact 7 segments, by migrating 6 segments worth of data. This results
in significant amplification. Compaction involves moving objects,
which in some cases is expensive in itself as well
(See https://github.com/scylladb/scylla/issues/3247).
This patch reduces amount of segment compactions in favor of doing
more eviction. It especially helps workloads in which LRU order
matches allocation order, in which case there will be no segment
compaction, and just eviction.
In perf_row_cache_update test case for large partition with lots of
rows, which simulates appending workload, I measured that for each new
object allocated, 2 need to be migrated, before the patch. After the
patch, only 0.003 objects are migrated. This reduces run time of
cache update part by 50%.
We are slightly underestimating the amount of memory we use. Now that
the chunked vector can exports its internal memory usage we can use that
directly.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
There are times in which we would like to estimate how much memory
a chunked_vector is using. We have two strategies to do it:
1) multiply the size by the size of the elements. That is wrong, because
the chunked_vector can allocate larger chunks in anticipation of more
elements to come.
2) multiply the number of chunks by 128kB. That is also wrong, because
the chunk_vector will not always allocate the entire chunk if there are
only a few elements in it.
The best way to deal with it is to allow the chunked_vector to exports
its current memory usage.
Signed-off-by: Glauber Costa <glauber@scylladb.com>
Currently reserve refill allocates segments repeatedly until the
reserve threhsold is met. If single segment allocation needs to
reclaim memory, it will ask the reclaimer for one segment. The
reclaimer could make better decisions if it knew the total number of
segments we try to allocate. In particular, it would not attempt to
compact any segment until it evicts total amount of memory first,
which may reduce the total amount of segment compactions during
refill.
This patch changes refill to increase reclamation step used by
allocate_segment() so that it matches the total amount of memory we
refill.
While the migration function should have enough information to obtain
the object size itself, the LSA logic needs to compute it as well.
IMR is going to make calculating object sizes more expensive, so by
providing the infromation to the migrator we can avoid some needless
operations.
It is non-trivial to get the size of an IMR object. However, the
standard allocator doesn't really need it and LSA can compute it itself
by asking the migrator.
Having migrators dynamically registered and deregistered opens a new
class of bugs. This patch adds some additional checks in the debug mode
with the hopes of catching any misuse early.
With the introduction of the new in-memory representation we will get
type- and schema-dependent migrators. Since there is no bound how many
times they can be created and destroyed it is better to be safe and
reuse registered migrator ids.
"Fixes a bug in partition_snapshot::merge_partition_versions(), which would not
attempt merging if the snapshot is attached to the latest version (in which
case _version is nullptr and _entry is != nullptr). This would cause
partition_version objects to accumulate if there was an older snapshot and it
went away before the latest snapshot. Versions will be removed when the whole
entry goes away (flush or eviction).
May cause performance problems.
Fixes #3402."
* 'tgrabiec/fix-merge_partition_versions' of github.com:tgrabiec/scylla:
mvcc: Test version merging when snapshots go away
anchorless_list: Make ranges conform to SinglePassRange
anchorless_list: Drop deprecated use of std::iterator
mvcc: Fix partition_snapshot::merge_partition_versions() to not leave latest versions unmerged
The two hash values, base and increment, used to produce indices for
setting bits in the filter, have been swapped in SSTables 3.0.
See CASSANDRA-8413 for details.
Signed-off-by: Vladimir Krivopalov <vladimir@scylladb.com>
* seastar 70aecca...ac02df7 (5):
> Merge "Prefix preprocessor definitions" from Jesse
> cmake: Do not enable warnings transitively
> posix: prevent unused variable warning
> build: Adjust DPDK options to fix compilation
> io_scheduler: adjust property names
DEBUG, DEFAULT_ALLOCATOR, and HAVE_LZ4_COMPRESS_DEFAULT macro
references prefixed with SEASTAR_. Some may need to become
Scylla macros.
They will be re-used for collecting encoding statistics which is needed
to write SSTables 3.0.
Part of #1969.
Signed-off-by: Vladimir Krivopalov <vladimir@scylladb.com>
