flat_mutation_reader_v2 was introduced in a pair of commits in 2021:
e3309322c3 "Clone flat_mutation_reader related classes into v2 variants"
08b5773c12 "Adapt flat_mutation_reader_v2 to the new version of the API"
as a replacement for flat_mutation_reader, using range_tombstone_change
instead of range_tombstone to represent represent range tombstones. See
those commits for more information.
The transition was incremental; the last use of the original
flat_mutation_reader was removed in 2022 in commit
026f8cc1e7 "db: Use mutation_partition_v2 in mvcc"
In turn, flat_mutation_reader was introduced in 2017 in commit
748205ca75 "Introduce flat_mutation_reader"
To transition from a mutation_reader that nested rows within
a partition in a separate stream, to a flat reader that streamed
partitions and rows in the same stream.
Here, we reclaim the original name and rename the awkward
flat_mutation_reader_v2 to mutation_reader.
Note that mutation_fragment_v2 remains since we still use the original
for compatibilty, sometimes.
Some notes about the transition:
- files were also renamed. In one case (flat_mutation_reader_test.cc), the
rename target already existed, so we rename to
mutation_reader_another_test.cc.
- a namespace 'mutation_reader' with two definitions existed (in
mutation_reader_fwd.hh). Its contents was folded into the mutation_reader
class. As a result, a few #includes had to be adjusted.
Closesscylladb/scylladb#19356
Fixes some typos as found by codespell run on the code.
In this commit, I was hoping to fix only comments, not user-visible alerts, output, etc.
Follow-up commits will take care of them.
Refs: https://github.com/scylladb/scylladb/issues/16255
Signed-off-by: Yaniv Kaul <yaniv.kaul@scylladb.com>
In mutation_reader_merger and clustering_order_reader_merger, the
operator()() is responsible for producing mutation fragments that will
be merged and pushed to the combined reader's buffer. Sometimes, it
might have to advance existing readers, open new and / or close some
existing ones, which requires calling a helper method and then calling
operator()() recursively.
In some unlucky circumstances, a stack overflow can occur:
- Readers have to be opened incrementally,
- Most or all readers must not produce any fragments and need to report
end of stream without preemption,
- There has to be enough readers opened within the lifetime of the
combined reader (~500),
- All of the above needs to happen within a single task quota.
In order to prevent such a situation, the code of both reader merger
classes were modified not to perform recursion at all. Most of the code
of the operator()() was moved to maybe_produce_batch which does not
recur if it is not possible for it to produce a fragment, instead it
returns std::nullopt and operator()() calls this method in a loop via
seastar::repeat_until_value.
A regression test is added.
Fixes: scylladb/scylladb#14415
Closes#14452
The series fixes the `make_nonforwardable` reader, it shouldn't emit `partition_end` for previous partition after `next_partition()` and `fast_forward_to()`
Fixes: #12249Closes#12978
* github.com:scylladb/scylladb:
flat_mutation_reader_test: cleanup, seastar::async -> SEASTAR_THREAD_TEST_CASE
make_nonforwardable: test through run_mutation_source_tests
make_nonforwardable: next_partition and fast_forward_to when single_partition is true
make_forwardable: fix next_partition
flat_mutation_reader_v2: drop forward_buffer_to
nonforwardable reader: fix indentation
nonforwardable reader: refactor, extract reset_partition
nonforwardable reader: add more tests
nonforwardable reader: no partition_end after fast_forward_to()
nonforwardable reader: no partition_end after next_partition()
nonforwardable reader: no partition_end for empty reader
row_cache: pass partition_start though nonforwardable reader
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>
Today, mutation_reader_merger drops unneeded readers in batches of 4,
meaning that the merger is having to keep the memory used by 3
unneeded readers in addition to the ones being currently read from.
As each may own a lot of memory, the combined effect of this waste,
coming from parallel reads, can potentially cause memory pressure.
This batching behavior was introduced in b524f96a74,
when readers had to be destroyed synchronously, as flat_mutation_reader
lacked an async close interface. But we have gone a long way since
then. Readers can be closed asynchronously and outstanding I/O
requests will be cancelled on close.
Now, we'll close readers as soon they're uneeded, one at a time,
using a continuation chain. If we're submitting close calls faster
than we can retire them, then we wait for their completion,
preventing memory usage from growing unbounded.
The benefit of this new approach will be very good when combining
disjoint readers, where only one is active at a time for producing
fragments. As soon as we're done with the current one, then it will
be closed allowing its memory to be released, before we move on
to the next reader that follows.
Refs #11040.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Closes#11167
