[daegyu94]

Description

ShardAllocator reads its shard topology with
std::atomic_load_explicit(&shards_) on every Allocate, Free and
UpdateAccess. libstdc++ implements this with a global mutex pool, so master
RPC workers serialize there even when they touch different shards.
UpdateAccess runs on every DFS read and PutEnd.

ShardList is an immutable snapshot that changes rarely (only on Init and
a growing ExpandShards via the serialized admin API), and old snapshots can
be kept until the allocator is destroyed. This PR therefore publishes it
through std::atomic<const ShardList*>, so lookups are a lock-free acquire
load, safe without shared_ptr reference counting. The ShardState objects
it points to remain shared_ptr, and behavior is unchanged.

Trade-off: since readers hold no reference, superseded snapshots are not
reclaimed. Each costs 16 bytes per shard, and one is added only when an
operator grows the shard count through the admin API, which sets a new total
(e.g. 64 → 128). Normal expansions therefore retain a few KiB in total; only
an unusual pattern of many single-shard expansions grows it quadratically. A
new master term creates a new allocator, so nothing accumulates across leader
changes. In return, every Allocate, Free and UpdateAccess stops taking
a process-wide mutex. std::atomic<std::shared_ptr> was not used because it
still updates the shared reference count on every load.

Microbenchmark (64 shards, median of 3 runs; local benchmark not included in
this PR; AMD Ryzen Threadripper 9960X, GCC 13, Release):

threads	UpdateAccess	Allocate+Free
1	21.6 → 37 Mops/s	3.8 → 4.4 Mops/s
16 (master default)	1.8 → 25 Mops/s	0.99 → 7.5 Mops/s

Note: this will conflict textually with #4018 on the shards_ declaration.

Suggested Merge Priority

• P2 — Normal: Routine bug fix, validated performance/compatibility improvement, or new feature for a confirmed use case

Module

• Mooncake Store (mooncake-store)

Type of Change

• Performance improvement

How Has This Been Tested?

Test commands:

./build/mooncake-store/tests/dfs_posix_test
./build/mooncake-store/tests/master_service_dfs_scenario_test
setarch $(uname -m) -R ./build-tsan/mooncake-store/tests/dfs_posix_test  # TSan

Test results:

• Unit tests pass

Both pass, also under ASan. TSan reports no races, including
ConcurrentExpansionAllocationAndEvictionRestore.

Checklist

• I have performed a self-review of my own code
• I have run pre-commit on the files changed in this PR and all hooks pass

AI Assistance Disclosure

• AI tools were used (specify below)

Claude Code assisted with analysis, implementation, testing and this
description. I reviewed every changed line.

[fcczzz]

LGTM