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Commit3bd8439

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Allow BufferAccessStrategy to limit pin count.
While pinning extra buffers to look ahead, users of strategies are indanger of using too many buffers. For some strategies, that means"escaping" from the ring, and in others it means forcing dirty data todisk very frequently with associated WAL flushing. Since external codehas no insight into any of that, allow individual strategy types toexpose a clamp that should be applied when deciding how many buffers topin 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
1 parentf956ecd commit3bd8439

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3 files changed

+48
-0
lines changed

3 files changed

+48
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lines changed

‎src/backend/storage/aio/read_stream.c‎

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@@ -419,6 +419,7 @@ read_stream_begin_relation(int flags,
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size_tsize;
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int16queue_size;
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int16max_ios;
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intstrategy_pin_limit;
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uint32max_pinned_buffers;
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Oidtablespace_id;
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SMgrRelationsmgr;
@@ -460,6 +461,10 @@ read_stream_begin_relation(int flags,
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max_pinned_buffers=Min(max_pinned_buffers,
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PG_INT16_MAX-io_combine_limit-1);
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/* Give the strategy a chance to limit the number of buffers we pin. */
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strategy_pin_limit=GetAccessStrategyPinLimit(strategy);
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max_pinned_buffers=Min(strategy_pin_limit,max_pinned_buffers);
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/* Don't allow this backend to pin more than its share of buffers. */
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if (SmgrIsTemp(smgr))
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LimitAdditionalLocalPins(&max_pinned_buffers);

‎src/backend/storage/buffer/freelist.c‎

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@@ -629,6 +629,48 @@ GetAccessStrategyBufferCount(BufferAccessStrategy strategy)
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returnstrategy->nbuffers;
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}
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/*
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* GetAccessStrategyPinLimit -- get cap of number of buffers that should be pinned
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*
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* When pinning extra buffers to look ahead, users of a ring-based strategy are
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* in danger of pinning too much of the ring at once while performing look-ahead.
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* For some strategies, that means "escaping" from the ring, and in others it
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* means forcing dirty data to disk very frequently with associated WAL
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* flushing. Since external code has no insight into any of that, allow
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* individual strategy types to expose a clamp that should be applied when
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* deciding on a maximum number of buffers to pin at once.
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*
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* Callers should combine this number with other relevant limits and take the
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* minimum.
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*/
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int
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GetAccessStrategyPinLimit(BufferAccessStrategystrategy)
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{
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if (strategy==NULL)
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returnNBuffers;
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switch (strategy->btype)
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{
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caseBAS_BULKREAD:
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/*
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* Since BAS_BULKREAD uses StrategyRejectBuffer(), dirty buffers
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* shouldn't be a problem and the caller is free to pin up to the
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* entire ring at once.
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*/
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returnstrategy->nbuffers;
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default:
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/*
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* Tell caller not to pin more than half the buffers in the ring.
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* This is a trade-off between look ahead distance and deferring
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* writeback and associated WAL traffic.
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*/
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returnstrategy->nbuffers /2;
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}
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}
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/*
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* FreeAccessStrategy -- release a BufferAccessStrategy object
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*

‎src/include/storage/bufmgr.h‎

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@@ -318,6 +318,7 @@ extern BufferAccessStrategy GetAccessStrategy(BufferAccessStrategyType btype);
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externBufferAccessStrategyGetAccessStrategyWithSize(BufferAccessStrategyTypebtype,
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intring_size_kb);
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externintGetAccessStrategyBufferCount(BufferAccessStrategystrategy);
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externintGetAccessStrategyPinLimit(BufferAccessStrategystrategy);
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externvoidFreeAccessStrategy(BufferAccessStrategystrategy);
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