6b415cfd4b
Commit 14bf09f447 added a single-chunk layout to `managed_bytes`, which makes the overhead of `managed_bytes` smaller in the common case of a small buffer.
But there was a bug in it. In the copy constructor of `managed_bytes`, a copy of a single-chunk `managed_bytes` is made single-chunk too.
But this is wrong, because the source of the copy and the target of the copy might have different preferred max contiguous allocation sizes.
In particular, if a `managed_bytes` of size between 13 kiB and 128 kiB is copied from the standard allocator into LSA, the resulting `managed_bytes` is a single chunk which violates LSA's preferred allocation size. (And therefore is placed by LSA in the standard allocator).
In other words, since Scylla 6.0, cache and memtable cells between 13 kiB and 128 kiB are getting allocated in the standard allocator rather than inside LSA segments.
Consequences of the bug:
1. Effective memory consumption of an affected cell is rounded up to the nearest power of 2.
2. With a pathological-enough allocation pattern (for example, one which somehow ends up placing a single 16 kiB memtable-owned allocation in every aligned 128 kiB span), memtable flushing could theoretically deadlock, because the allocator might be too fragmented to let the memtable grow by another 128 kiB segment, while keeping the sum of all allocations small enough to avoid triggering a flush. (Such an allocation pattern probably wouldn't happen in practice though).
3. It triggers a bug in reclaim which results in spurious allocation failures despite ample evictable memory.
There is a path in the reclaimer procedure where we check whether reclamation succeeded by checking that the number of free LSA segments grew.
But in the presence of evictable non-LSA allocations, this is wrong because the reclaim might have met its target by evicting the non-LSA allocations, in which case memory is returned directly to the standard allocator, rather than to the pool of free segments.
If that happens, the reclaimer wrongly returns `reclaimed_nothing` to Seastar, which fails the allocation.
Refs (possibly fixes) https://github.com/scylladb/scylladb/issues/21072
Fixes https://github.com/scylladb/scylladb/issues/22941
Fixes https://github.com/scylladb/scylladb/issues/22389
Fixes https://github.com/scylladb/scylladb/issues/23781
This is a regression fix, should be backported to all affected releases.
Closes scylladb/scylladb#23782
* github.com:scylladb/scylladb:
managed_bytes_test: add a reproducer for #23781
managed_bytes: in the copy constructor, respect the target preferred allocation size
Scylla in-source tests.
For details on how to run the tests, see docs/dev/testing.md
Shared C++ utils, libraries are in lib/, for Python - pylib/
alternator - Python tests which connect to a single server and use the DynamoDB API unit, boost, raft - unit tests in C++ cqlpy - Python tests which connect to a single server and use CQL topology* - tests that set up clusters and add/remove nodes cql - approval tests that use CQL and pre-recorded output rest_api - tests for Scylla REST API Port 9000 scylla-gdb - tests for scylla-gdb.py helper script nodetool - tests for C++ implementation of nodetool
If you can use an existing folder, consider adding your test to it. New folders should be used for new large categories/subsystems, or when the test environment is significantly different from some existing suite, e.g. you plan to start scylladb with different configuration, and you intend to add many tests and would like them to reuse an existing Scylla cluster (clusters can be reused for tests within the same folder).
To add a new folder, create a new directory, and then
copy & edit its suite.ini.