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Improve buffer manager API for backend pin limits.

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Previously the support functions assumed that the caller needed one pin
to make progress, and could optionally use some more, allowing enough
for every connection to do the same.  Add a couple more functions for
callers that want to know:

* what the maximum possible number could be, irrespective of currently
  held pins, for space planning purposes

* how many additional pins they could acquire right now, without the
  special case allowing one pin, for callers that already hold pins and
  could already make progress even if no extra pins are available

The pin limit logic began in commit 31966b15.  This refactoring is
better suited to read_stream.c, which will be adjusted to respect the
remaining limit as it changes over time in a follow-up commit.  It also
computes MaxProportionalPins up front, to avoid performing divisions
whenever a caller needs to check the balance.

Reviewed-by: Andres Freund <andres@anarazel.de> (earlier versions)
Discussion: https://postgr.es/m/CA%2BhUKGK_%3D4CVmMHvsHjOVrK6t4F%3DLBpFzsrr3R%2BaJYN8kcTfWg%40mail.gmail.com
This commit is contained in:
Thomas Munro
2025-03-14 15:10:43 +13:00
parent 7c99dc587a
commit 01261fb078
3 changed files with 75 additions and 25 deletions
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4
src/include/storage/bufmgr.h
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@@ -290,6 +290,10 @@ extern bool HoldingBufferPinThatDelaysRecovery(void);
extern bool BgBufferSync(struct WritebackContext *wb_context);
extern uint32 GetPinLimit(void);
extern uint32 GetLocalPinLimit(void);
extern uint32 GetAdditionalPinLimit(void);
extern uint32 GetAdditionalLocalPinLimit(void);
extern void LimitAdditionalPins(uint32 *additional_pins);
extern void LimitAdditionalLocalPins(uint32 *additional_pins);