The querier cache refuses to cache queriers that read in reverse. These
queriers are also not closed, with the caller having no way to determine
whether the querier it just moved into `insert()` needs a close
afterwards or not, requiring a `close()` on the moved-from querier just
to be sure.
Avoid this by consistently closing all passed-in queriers, including
those the cache refuses to save. For this, the internal
`insert_querier()` methods has to be made a member to be able to use the
closing gate.
Close the _closing_gate to wait on background
close of dropped queries, and close all remaining queriers.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Make sure to close the dropped querier before it's destroyed.
The operation is moved to the background so not to penelize
the common path.
A following patch will add a querier_cache::close() method
that will close _closing_gate to wait on the querier close
(among other things it needs to wait on :)).
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
In preparation to closing the querier in the background
before dropping it.
With that, stats need not be passed as a parameter,
but rather the _stats member can be used directly.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Depening on the variant _reader contents, either close
the reader or unregister the inactive reader and close it.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
We don't own _source therefore do not close it.
That said, we still need to make sure that the reversing reader
itself is closed to calm down the check when it's destroyed.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
"
Currently inactive readers are stored in two different places:
* reader concurrency semaphore
* querier cache
With the latter registering its inactive readers with the former. This
is an unnecessarily complex (and possibly surprising) setup that we want
to move away from. This series solves this by moving the responsibility
if storing of inactive reads solely to the reader concurrency semaphore,
including all supported eviction policies. The querier cache is now only
responsible for indexing queriers and maintaining relevant stats.
This makes the ownership of the inactive readers much more clear,
hopefully making Benny's work on introducing close() and abort() a
little bit easier.
Tests: unit(release, debug:v1)
"
* 'unify-inactive-readers/v2' of https://github.com/denesb/scylla:
reader_concurrency_semaphore: store inactive readers directly
querier_cache: store readers in the reader concurrency semaphore directly
querier_cache: retire memory based cache eviction
querier_cache: delegate expiry to the reader_concurrency_semaphore
reader_concurrency_semaphore: introduce ttl for inactive reads
querier_cache: use new eviction notify mechanism to maintain stats
reader_concurrency_semaphore: add eviction notification facility
reader_concurrency_semaphore: extract evict code into method evict()
The querier cache has a memory limit it enforces on cached queriers. For
determining how much memory each querier uses, it currently uses
`flat_mutation_reader::buffer_size()`. However, we now have a much more
complete accounting of the memory each read consumes, in the form of the
reader permit, which also happens to be handy in the queriers. So use it
instead of the not very well maintained `buffer_size()`.
Currently, we cannot select more than 2^32 rows from a table because we are limited by types of
variables containing the numbers of rows. This patch changes these types and sets new limits.
The new limits take effect while selecting all rows from a table - custom limits of rows in a result
stay the same (2^32-1).
In classes which are being serialized and used in messaging, in order to be able to process queries
originating from older nodes, the top 32 bits of new integers are optional and stay at the end
of the class - if they're absent we assume they equal 0.
The backward compatibility was tested by querying an older node for a paged selection, using the
received paging_state with the same select statement on an upgraded node, and comparing the returned
rows with the result generated for the same query by the older node, additionally checking if the
paging_state returned by the upgraded node contained new fields with correct values. Also verified
if the older node simply ignores the top 32 bits of the remaining rows number when handling a query
with a paging_state originating from an upgraded node by generating and sending such a query to
an older node and checking the paging_state in the reply(using python driver).
Fixes#5101.
We want to switch from using a single limit to a dual soft/hard limit.
As a first step we switch the limit field of `query_class_config` to use
the recently introduced type for this. As this field has a single user
at the moment -- reverse queries (and not a lot of propagation) -- we
update it in this same patch to use the soft/hard limit: warn on
reaching the soft limit and abort on the hard limit (the previous
behaviour).
The querier cache expects all querier objects it stores to have certain
methods. To avoid accessing these via `std::visit()` (the querier object
is stored in an `std::variant`), we move all the stuff that is common to
all querier types into a base class. The querier cache now accesses the
members via a reference to this common base. Additionally the variant is
eliminated completely and the cache entry stores an
`std::unique_ptr<querier_base>` instead.
Tests: unit(dev)
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <20200603152544.83704-1-bdenes@scylladb.com>
Seastar recently lost support for the experimental Concepts Technical
Specification (TS) and gained support for C++20 concepts. Re-enable
concepts in Scylla by updating our use of concepts to the C++20
standard.
This change:
- peels off uses of the GCC6_CONCEPT macro
- removes inclusions of <seastar/gcc6-concepts.hh>
- replaces function-style concepts (no longer supported) with
equation-style concepts
- semicolons added and removed as needed
- deprecated std::is_pod replaced by recommended replacement
- updates return type constraints to use concepts instead of
type names (either std::same_as or std::convertible_to, with
std::same_as chosen when possible)
No attempt is made to improve the concepts; this is a specification
update only.
Message-Id: <20200531110254.2555854-1-avi@scylladb.com>
Currently the `querier_cache` is passed a semaphore during its
construction and it uses this semaphore to do all the inactive reader
registering/unregistering. This is inaccurate as in theory cached reads
could belong to different semaphores (although currently this is not yet
the case). As all queriers store a valid permit now, use this
permit to obtain the semaphore the querier is associated with, and
register the inactive read with this semaphore.
In preparation of a valid permit being required to be passed to all
mutation sources, create a permit before creating the shard readers and
pass it to the mutation source when doing so. The permit is also
persisted in the `shard_mutation_querier` object when saving the reader,
which is another forward looking change, to allow the querier-cache to
use it to obtain the semaphore the read is actually registered with.
In preparation of a valid permit being required to be passed to all
mutation sources, also add a permit to the querier object, which is then
passed to the source when it is used to create a reader.
The constructor of `read_command` is used both by IDL and clients in the
code. However, this constructor has a parameter that is not used by IDL:
`read_timestamp`. This requires that this parameter is the very last in
the list and that new parameters that are used by IDL are added before
it. One such new parameter was `bool is_first_page`. Adding this
parameter right before the read timestamp one created a situation where
the last parameter (read_timestamp) implicitly converts to the one
before it (is_first_page). This means that some call sites passing
`read_timestamp` were now silently converting this to `is_first_page`,
effectively dropping the timestamp.
This patch aims to rectify this, while also avoiding similar accidents
in the future, by making `is_first_page` a `bool_class` which doesn't
have any implicit convertions defined. This change does not break the
ABI as `bool_class` is also sent as a `bool` on the wire.
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Tests: unit(dev)
Message-Id: <20200422073657.87241-1-bdenes@scylladb.com>
Now that #3158 is fixed, we can move the consumer to its place after
the `compaction_mutation_state::start_new_page()` call. No need to keep
it as `std::unique_ptr<>`.
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <20200310185147.207665-1-bdenes@scylladb.com>
If the reversing requires more memory than the limit, the read is
aborted. All users are updated to get a meaningful limit, from the
respective table object, with the exception of tests of course.
Currently reverse reads just pass a flag to
`flat_mutation_reader::consume()` to make the read happen in reverse.
This is deceptively simple and streamlined -- while in fact behind the
scenes a reversing reader is created to wrap the reader in question to
reverse partitions, one-by-one.
This patch makes this apparent by exposing the reversing reader via
`make_reversing_reader()`. This now makes how reversing works more
apparent. It also allows for more configuration to be passed to the
reversing reader (in the next patches).
This change is forward compatible, as in time we plan to add reversing
support to the sstable layer, in which case the reversing reader will
go.
The former was never really more than a reader_permit with one
additional method. Currently using it doesn't even save one from any
includes. Now that readers will be using reader_permit we would have to
pass down both to mutation_source. Instead get rid of
reader_resource_tracker and just use reader_permit. Instead of making it
a last and optional parameter that is easy to ignore, make it a
first class parameter, right after schema, to signify that permits are
now a prominent part of the reader API.
This -- mostly mechanical -- patch essentially refactors mutation_source
to ask for the reader_permit instead of reader_resource_tracking and
updates all usage sites.
Recently we have seen a case where the population stat of the cache was
corrupt, either due to misaccounting or some more serious corruption.
When debugging something like that it would have been useful to know how
many items have been inserted to the cache. I also believe that such a
counter could be useful generally as well.
Refs: #4918
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <20190924083429.43038-1-bdenes@scylladb.com>
As we are about to add multiple sources of evictable readers, we need a
more scalable solution than a single functor being passed that opaquely
evicts a reader when called.
Add a generic way to register and unregister evictable (inactive)
readers to the semaphore. The readers are expected to be registered when
they become evictable and are expected to be unregistered when they
cease to become evictable. The semaphore might evict any reader that is
registered to it, when it sees fit.
This also solves the problem of notifying the semaphore when new readers
become evictable. Previously there was no such mechanism, and the
semaphore would only evict any such new readers when a new permit was
requested from it.
Currently timeout is opt-in, that is, all methods that even have it
default it to `db::no_timeout`. This means that ensuring timeout is used
where it should be is completely up to the author and the reviewrs of
the code. As humans are notoriously prone to mistakes this has resulted
in a very inconsistent usage of timeout, many clients of
`flat_mutation_reader` passing the timeout only to some members and only
on certain call sites. This is small wonder considering that some core
operations like `operator()()` only recently received a timeout
parameter and others like `peek()` didn't even have one until this
patch. Both of these methods call `fill_buffer()` which potentially
talks to the lower layers and is supposed to propagate the timeout.
All this makes the `flat_mutation_reader`'s timeout effectively useless.
To make order in this chaos make the timeout parameter a mandatory one
on all `flat_mutation_reader` methods that need it. This ensures that
humans now get a reminder from the compiler when they forget to pass the
timeout. Clients can still opt-out from passing a timeout by passing
`db::no_timeout` (the previous default value) but this will be now
explicit and developers should think before typing it.
There were suprisingly few core call sites to fix up. Where a timeout
was available nearby I propagated it to be able to pass it to the
reader, where I couldn't I passed `db::no_timeout`. Authors of the
latter kind of code (view, streaming and repair are some of the notable
examples) should maybe consider propagating down a timeout if needed.
In the test code (the wast majority of the changes) I just used
`db::no_timeout` everywhere.
Tests: unit(release, debug)
Signed-off-by: Botond Dénes <bdenes@scylladb.com>
Message-Id: <1edc10802d5eb23de8af28c9f48b8d3be0f1a468.1536744563.git.bdenes@scylladb.com>
The querier to be used for saving shard readers belonging to a
multishard range scan. This querier doesn't provide a `consume_page`
method as it doesn't support reading from it directly. It is more
of a storage to allow caching the reader and any objects it depends on.
In the next patch a querier will be added that reads multiple ranges as
opposed to a single range that data and mutation queriers read.
To keep `querier_cache` code seamless regarding this difference change all
range-matching logic to work in terms of `dht::partition_ranges_view`.
This allows for cheap and seamless way of having a single code-base for
the insert/lookup logic. Code actually matching ranges is updated to be
able to handle both singular and multi-ranges while maintaining backward
compatibility.
Instead of hiding what compaction method the querier uses (and only
expose it via rejecting 'can_be_used_for_page()`) make it very explicit
that these are really two different queriers. This allows using
different indexes for the two queriers in `querier_cache` and
eliminating the possibility of picking up a querier with the wrong
compaction method (read kind).
This also makes it possible to add new querier type(s) that suit the
multishard-query's needs without making a confusing mess of `querier` by
making it a union of all querying logic.
Splitting the queriers this way changes what happens when a lookup finds
a querier of the wrong kind (e.g. emit_only_live::yes for an
emit_only_live::no command). As opposed to dropping the found (but
wrong) querier the querier will now simply not be found by the lookup.
This is a result of using separate search indexes for the different
mutation kinds. This change should have no practical implications.
Splitting is done by making querier templated on `emit_only_live_rows`.
It doesn't make sense to duplicate the entire querier as the two share
99% of the code.
In preparation of the now single querier being split into multiple more
specialized ones. Make it possible for the multiple queriers sharing the
same implementation. Also, the code can now be reused by outside code as
well, not just queriers.
So that they can be used for multiple querier classes easily, without
inheritance. The functions are not visible from the header.
Also update the comments on `querier` to w.r.t. the disappeared
checking functions. Change the language to be more general. In practice
these checks are never done by client code, instead they are done by the
`querier_cache`.
In preparations for introducing support multiple entry types in the
querier_cache move all insert/lookup related logic into free functions.
Later these functions will be templated so they can handle multiple
entry types with the same code.
Requiring the caller of lookup() to pass in a `create_fun()` was not
such a good idea in hindsight. It leads to awkward call sites and even
more awkward code when trying to find out whether the lookup was
successfull or not.
Returning an optional gives calling code much more flexibility and makes
the code cleaner.
Currently querier_cache uses a `std::unordered_map<utils::UUID, querier>`
to store cache entries and an `std::list<meta_entry>` to store meta
information about the querier entries, like insertion order, expiry
time, etc.
All cache eviction algorithms use the meta-entry list to evict entries
in reverse insertion order (LRU order). To make this possible
meta-entries keep an iterator into the entry map so that given a
meta-entry one can easily erase the querier entry. This however poses a
problem as std::unordered_map can possibly invalidate all its iterators
when new items are inserted. This is use-after-free waiting to happen.
Another disadvantages of the current solution is that it requires the
meta-entry to use a weak pointer to the querier entry so that in case
that is removed (as a result of a successful lookup) it doesn't try to
access it. This has an impact on all cache eviction algorithms as they
have to be prepared to deal with stale meta-entries. Stale meta-entries
also unnecesarily consume memory.
To solve these problems redesign how querier_cache stores entries
completely. Instead of storing the entries in an `std::unordered_map`
and storing the meta-entries in an `std::list`, store the entries in an
`std::list` and an intrusive-map (index) for lookups. This new design
has severeal advantages over the old one:
* The entries will now be in insert order, so eviction strategies can
work on the entry list itself, no need to involve additional data
structures for this.
* All data related to an entry is stored in one place, no data
duplication.
* Removing an entry automatically removes it from the index as intrusive
containers support auto unlink. This means there is no need to store
iterators for long terms, risking use-after-free when the container
invalidates it's iterators.
Additional changes:
* Modify eviction strategies so that they work with the `entry`
interface rather than the stored value directly.
Ref #3424
The querier provides a `matches(const nonwrapping_range&)` member to
allow for checking whether a range matches that with which the querier
was originally created. The check for match is more lax than a strict
equality check as ranges are shrunk query progresses.
Because of this the above member only checked that one of the bounds of
the examined ranges matches. This is adequate as for this purpose
because, in the context of a single query, it is guaranteed that no
two read requests to the same replica will have overlapping range.
However Avi pointed out in a recent, related review, that this check can
be made a little more strict by requiring that the end-bounds of the
two ranges *always* matches, instead of allowing any of the bounds to
match.
Add the following counters:
(1) querier_cache_lookups
(2) querier_cache_misses
(3) querier_cache_drops
(4) querier_cache_time_based_evictions
(5) querier_cache_resource_based_evictions
(6) querier_cache_memory_based_evictions
(6) querier_cache_population
(1) counts the total number of querier cache lookups. Not all
page-fetches will result in a querier lookup. For example the first page
of a query will not do a lookup as there was no previous page to reuse
the querier from. The second, and all subsequent pages however should
attempt to reuse the querier from the previous page.
(2) counts the subset of (1) where the read have missed the querier
cache (failed to find a matching saved querier).
(3) counts the subset of (1) where the querier was recalled and dropped
immediately. This can happen for example if the querier was at the wrong
position.
(4) counts the cached queriers that were evicted due to their TTL
expiring.
(5) counts the cached queriers that were evicted due to reader-resource
(those limited by reader-concurrency limits) shortage.
(6) counts the cached queriers that were evicted due to reaching the
cache's memory limits (currently set to 4% of the shards' memory).
(7) is the current number of entries in the cache
Note:
* The count of cache hits can be derived from these counters as
(1) - (2).
* cache_drop (3) also implies a cache hit (see above). This means that
the number of actually reused queriers is:
(1) - (2) - (3)
