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Allow BufferAccessStrategy to limit pin count.

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While pinning extra buffers to look ahead, users of strategies are in
danger of using too many buffers.  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 <andres@anarazel.de>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/CAAKRu_aJXnqsyZt6HwFLnxYEBgE17oypkxbKbT1t1geE_wvH2Q%40mail.gmail.com
This commit is contained in:
Thomas Munro
2024-04-06 22:58:48 +13:00
parent f956ecd035
commit 3bd8439ed6
3 changed files with 48 additions and 0 deletions
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5
src/backend/storage/aio/read_stream.c
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@@ -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);