From e34087c5248b614b7b73b053ccb687be4f78d553 Mon Sep 17 00:00:00 2001
From: Peter Dillinger <peterd@meta.com>
Date: Wed, 30 Oct 2024 20:37:28 -0700
Subject: [PATCH] Add a temporary hook for custom yielding in long-running op
 (#13103)
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Summary:
This is a simplified version of https://github.com/facebook/rocksdb/issues/13096, which called for a way to hook into long-running loops completely within RocksDB to change their thread priority (or similar). The current prime hook point is `DBIter::FindNextUserEntryInternal` likely because of iterating over tombstones.

This is implemented using the weak symbol hack for ease of back-porting/patching, and while we get to know potential future requirements better for integration into the public API. (Consider potential relationships to `Env::GetThreadStatusUpdater()` and `TransactionDBMutexFactory`.)

Pull Request resolved: https://github.com/facebook/rocksdb/pull/13103

Test Plan:
Performance validated with db_bench and DEBUG_LEVEL=0: `./db_bench --benchmarks=fillseq,deleterandom,readseq[-X100] --value_size=1 --num=1000000`

No consistent difference seen; variances likely in how DB / executable / memory were laid out.

```
With an empty hook:
readseq [AVG    100 runs] : 1753018 (± 8850) ops/sec;   28.4 (± 0.1) MB/sec
readseq [MEDIAN 100 runs] : 1763746 ops/sec;   28.6 MB/sec
(recompile)
readseq [AVG    100 runs] : 1789019 (± 10260) ops/sec;   29.0 (± 0.2) MB/sec
readseq [MEDIAN 100 runs] : 1801849 ops/sec;   29.2 MB/sec

Base:
readseq [AVG    100 runs] : 1772196 (± 8240) ops/sec;   28.7 (± 0.1) MB/sec
readseq [MEDIAN 100 runs] : 1780453 ops/sec;   28.9 MB/sec
(recompile)
readseq [AVG    100 runs] : 1777637 (± 7613) ops/sec;   28.8 (± 0.1) MB/sec
readseq [MEDIAN 100 runs] : 1786657 ops/sec;   29.0 MB/sec

With a functional hook (count number of calls into it):
readseq [AVG    100 runs] : 1796733 (± 8854) ops/sec;   29.1 (± 0.1) MB/sec
readseq [MEDIAN 100 runs] : 1804690 ops/sec;   29.3 MB/sec
RocksDbThreadYield: 126915800
(recompile)
readseq [AVG    100 runs] : 1775371 (± 10529) ops/sec;   28.8 (± 0.2) MB/sec
readseq [MEDIAN 100 runs] : 1789046 ops/sec;   29.0 MB/sec
RocksDbThreadYield: 126977000

Base:
readseq [AVG    100 runs] : 1773071 (± 10657) ops/sec;   28.7 (± 0.2) MB/sec
readseq [MEDIAN 100 runs] : 1783414 ops/sec;   28.9 MB/sec
(recompile)
readseq [AVG    100 runs] : 1750852 (± 10184) ops/sec;   28.4 (± 0.2) MB/sec
readseq [MEDIAN 100 runs] : 1763587 ops/sec;   28.6 MB/sec
```

Reviewed By: george-reynya

Differential Revision: D65235379

Pulled By: pdillinger

fbshipit-source-id: 7829e4cc25a56d4c1801b8adf9c7f7aa49ab7aca
---
 db/db_impl/db_impl.cc |  2 ++
 db/db_iter.cc         |  2 ++
 port/port.h           | 16 ++++++++++++++++
 3 files changed, 20 insertions(+)

diff --git a/db/db_impl/db_impl.cc b/db/db_impl/db_impl.cc
index a9cde476ac..48f6529bee 100644
--- a/db/db_impl/db_impl.cc
+++ b/db/db_impl/db_impl.cc
@@ -3230,6 +3230,8 @@ Status DBImpl::MultiGetImpl(
       s = Status::Aborted();
       break;
     }
+    // This could be a long-running operation
+    ROCKSDB_THREAD_YIELD_HOOK();
   }
 
   // Post processing (decrement reference counts and record statistics)
diff --git a/db/db_iter.cc b/db/db_iter.cc
index 97f6f7a076..49537f7011 100644
--- a/db/db_iter.cc
+++ b/db/db_iter.cc
@@ -581,6 +581,8 @@ bool DBIter::FindNextUserEntryInternal(bool skipping_saved_key,
     } else {
       iter_.Next();
     }
+    // This could be a long-running operation due to tombstones, etc.
+    ROCKSDB_THREAD_YIELD_HOOK();
   } while (iter_.Valid());
 
   valid_ = false;
diff --git a/port/port.h b/port/port.h
index 13aa56d47b..141716e5b9 100644
--- a/port/port.h
+++ b/port/port.h
@@ -19,3 +19,19 @@
 #elif defined(OS_WIN)
 #include "port/win/port_win.h"
 #endif
+
+#ifdef OS_LINUX
+// A temporary hook into long-running RocksDB threads to support modifying their
+// priority etc. This should become a public API hook once the requirements
+// are better understood.
+extern "C" void RocksDbThreadYield() __attribute__((__weak__));
+#define ROCKSDB_THREAD_YIELD_HOOK() \
+  {                                 \
+    if (RocksDbThreadYield) {       \
+      RocksDbThreadYield();         \
+    }                               \
+  }
+#else
+#define ROCKSDB_THREAD_YIELD_HOOK() \
+  {}
+#endif