From 1d88127b78b11e7a1fcb9d4e5a21227ada32d262 Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Thu, 4 Apr 2024 20:31:26 +1300 Subject: [PATCH v12 4/6] Allow BufferAccessStrategy to limit pin count. When pinning extra buffers to look ahead, users of a strategy are in danger of pinning a lot of the buffers in the ring, or even more than the ring size. For some strategies, that means "escaping" from the ring, and in others it means forcing dirty data to disk very frequently with associated WAL flushing. Since external code has no insight into any of that, allow individual strategy types to expose a clamp that should be applied when deciding how many buffers to pin at once. Reviewed-by: Andres Freund Reviewed-by: Melanie Plageman Discussion: https://postgr.es/m/CAAKRu_aJXnqsyZt6HwFLnxYEBgE17oypkxbKbT1t1geE_wvH2Q%40mail.gmail.com --- src/backend/storage/aio/read_stream.c | 5 ++++ src/backend/storage/buffer/freelist.c | 42 +++++++++++++++++++++++++++ src/include/storage/bufmgr.h | 1 + 3 files changed, 48 insertions(+) diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c index e5b64238f21..164049f1474 100644 --- a/src/backend/storage/aio/read_stream.c +++ b/src/backend/storage/aio/read_stream.c @@ -419,6 +419,7 @@ read_stream_begin_relation(int flags, size_t size; int16 queue_size; int16 max_ios; + int strategy_pin_limit; uint32 max_pinned_buffers; Oid tablespace_id; SMgrRelation smgr; @@ -460,6 +461,10 @@ read_stream_begin_relation(int flags, max_pinned_buffers = Min(max_pinned_buffers, PG_INT16_MAX - io_combine_limit - 1); + /* Give the strategy a chance to limit the number of buffers we pin. */ + strategy_pin_limit = GetAccessStrategyPinLimit(strategy); + max_pinned_buffers = Min(strategy_pin_limit, max_pinned_buffers); + /* Don't allow this backend to pin more than its share of buffers. */ if (SmgrIsTemp(smgr)) LimitAdditionalLocalPins(&max_pinned_buffers); diff --git a/src/backend/storage/buffer/freelist.c b/src/backend/storage/buffer/freelist.c index 3611357fa30..d8096d2ee4f 100644 --- a/src/backend/storage/buffer/freelist.c +++ b/src/backend/storage/buffer/freelist.c @@ -629,6 +629,48 @@ GetAccessStrategyBufferCount(BufferAccessStrategy strategy) return strategy->nbuffers; } +/* + * GetAccessStrategyPinLimit -- get cap of number of buffers that should be pinned + * + * When pinning extra buffers to look ahead, users of a ring-based strategy are + * in danger of pinning too much of the ring at once while performing look-ahead. + * For some strategies, that means "escaping" from the ring, and in others it + * means forcing dirty data to disk very frequently with associated WAL + * flushing. Since external code has no insight into any of that, allow + * individual strategy types to expose a clamp that should be applied when + * deciding on a maximum number of buffers to pin at once. + * + * Callers should combine this number with other relevant limits and take the + * minimum. + */ +int +GetAccessStrategyPinLimit(BufferAccessStrategy strategy) +{ + if (strategy == NULL) + return NBuffers; + + switch (strategy->btype) + { + case BAS_BULKREAD: + + /* + * Since BAS_BULKREAD uses StrategyRejectBuffer(), dirty buffers + * shouldn't be a problem and the caller is free to pin up to the + * entire ring at once. + */ + return strategy->nbuffers; + + default: + + /* + * Tell call not to pin more than half the buffers in the ring. + * This is a trade-off between look ahead distance and deferring + * writeback and associated WAL traffic. + */ + return strategy->nbuffers / 2; + } +} + /* * FreeAccessStrategy -- release a BufferAccessStrategy object * diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h index f380f9d9a6c..07ba1a60502 100644 --- a/src/include/storage/bufmgr.h +++ b/src/include/storage/bufmgr.h @@ -318,6 +318,7 @@ extern BufferAccessStrategy GetAccessStrategy(BufferAccessStrategyType btype); extern BufferAccessStrategy GetAccessStrategyWithSize(BufferAccessStrategyType btype, int ring_size_kb); extern int GetAccessStrategyBufferCount(BufferAccessStrategy strategy); +extern int GetAccessStrategyPinLimit(BufferAccessStrategy strategy); extern void FreeAccessStrategy(BufferAccessStrategy strategy); -- 2.44.0