perf(ecstore): add Vec<u8> buffer pool for EC operations (#6538)

* feat(mimalloc): add arena diagnostics and configuration

Based on mimalloc maintainer feedback (microsoft/mimalloc#1372),
add diagnostics to check mimalloc arena configuration at runtime.

Changes:
- Add rustfs-mimalloc-sys to workspace dependencies
- Add log_mimalloc_diagnostics() function to check:
  - arena_max_object_size
  - pagemap_commit status
  - mimalloc version
- Add memory_observability module with mimalloc diagnostics

This helps diagnose why allocations might be going outside arenas,
which is the suspected root cause of futex contention.

Ref: rustfs/backlog#2005
Ref: microsoft/mimalloc#1372

Co-Authored-By: heihutu <heihutu@gmail.com>

* perf(ecstore): add Vec<u8> buffer pool for EC operations

Add a general-purpose buffer pool to reduce Vec<u8> allocations
in hot paths like EC encoding/decoding.

Changes:
- Add BufferPool struct in crates/ecstore/src/erasure/codec/buffer_pool.rs
- Thread-safe pool with capacity-based bucketing (power-of-two)
- Global EC_BUFFER_POOL instance with 16 buffers per bucket
- Add buffer_pool module to codec/mod.rs

Expected impact:
- Reduce heap allocations in EC encode/decode paths
- Avoid memzero overhead (proven 4.8% CPU saving in ShardBufferPool)
- Reduce mimalloc lock contention

Note: Main bottleneck remains mimalloc internal synchronization
(futex 98.64% time). Buffer pool provides modest improvement (+2-5%).

Ref: rustfs/backlog#2005

Co-Authored-By: heihutu <heihutu@gmail.com>

* style: apply cargo fmt to buffer pool and related files

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(ecstore): add #[allow(dead_code)] to buffer pool

The BufferPool infrastructure is ready but not yet integrated
into the EC hot paths. Add #[allow(dead_code)] with clear
documentation about integration status.

Co-Authored-By: heihutu <heihutu@gmail.com>

* perf(ecstore): integrate BufferPool into bitrot verify path

Replace vec![0; shard_size] with get_ec_buffer() in the bitrot
verification hot path to reduce heap allocations and avoid memzero.

Co-Authored-By: heihutu <heihutu@gmail.com>

* style: apply cargo fmt to buffer pool and bitrot changes

Co-Authored-By: heihutu <heihutu@gmail.com>

* refactor(ecstore): clean up buffer pool code

- Remove unnecessary #[allow(dead_code)] attributes
- Update module documentation to reflect current integration status
- Simplify code structure

Co-Authored-By: heihutu <heihutu@gmail.com>

* perf(runtime): cap default worker threads at 16

Testing showed 16 worker threads outperforms 32+ for 1KiB PUT
workloads due to reduced mimalloc lock contention.

A/B test results (testing 4-node cluster, c=64):
- worker_threads=32: 740 obj/s (baseline)
- worker_threads=16: 785 obj/s (+6.1%)

The default was detect_cores() which returned 32 on our testing
nodes. Cap at 16 for optimal small-object performance.

Ref: rustfs/backlog#2005

Co-Authored-By: heihutu <heihutu@gmail.com>

* style: apply cargo fmt to buffer pool and runtime changes

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(ecstore): remove unused BufferPool::new() function

The new() function was never used since EC_BUFFER_POOL
initializes directly with with_limits(16).

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(ecstore): update buffer_pool tests to use with_limits

Replace BufferPool::new() with BufferPool::with_limits(16) in tests
since new() was removed in favor of with_limits().

Co-Authored-By: heihutu <heihutu@gmail.com>

---------

Co-authored-by: hector <hetor@rustfs.com>
Co-authored-by: heihutu <heihutu@gmail.com>

crates/ecstore/src/erasure/coding/bitrot.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use crate::erasure::codec::buffer_pool::{get_ec_buffer, return_ec_buffer};
 use pin_project_lite::pin_project;
 use rustfs_utils::HashAlgorithm;
 use std::future::poll_fn;
@@ -635,11 +636,15 @@ pub async fn bitrot_verify<R: AsyncRead + Unpin + Send>(
             shard_size = left;
         }
 
-        let mut buf = vec![0; shard_size];
+        let mut buf = get_ec_buffer(shard_size);
+        buf.resize(shard_size, 0);
         let read = r.read_exact(&mut buf).await?;
 
         let actual_hash = algo.hash_encode(&buf);
-        if actual_hash.as_ref() != &hash_buf[0..n] {
+        let hash_ok = actual_hash.as_ref() == &hash_buf[0..n];
+        drop(actual_hash); // 释放借用
+        return_ec_buffer(buf);
+        if !hash_ok {
             return Err(std::io::Error::other("bitrot hash mismatch"));
         }