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Introduce io_max_combine_limit.

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The existing io_combine_limit can be changed by users.  The new
io_max_combine_limit is fixed at server startup time, and functions as a
silent clamp on the user setting.  That in itself is probably quite
useful, but the primary motivation is:

aio_init.c allocates shared memory for all asynchronous IOs including
some per-block data, and we didn't want to waste memory you'd never used
by assuming they could be up to PG_IOV_MAX.  This commit already halves
the size of 'AioHandleIov' and 'AioHandleData'.  A follow-up commit can
now expand PG_IOV_MAX without affecting that.

Since our GUC system doesn't support dependencies or cross-checks
between GUCs, the user-settable one now assigns a "raw" value to
io_combine_limit_guc, and the lower of io_combine_limit_guc and
io_max_combine_limit is maintained in io_combine_limit.

Reviewed-by: Andres Freund <andres@anarazel.de> (earlier version)
Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com
This commit is contained in:
Thomas Munro
2025-03-19 11:40:56 +13:00
parent 17d8bba6da
commit 10f6646847
8 changed files with 72 additions and 15 deletions
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17
src/backend/storage/aio/aio_init.c
View File

@@ -18,6 +18,7 @@
#include "storage/aio.h"
#include "storage/aio_internal.h"
#include "storage/aio_subsys.h"
#include "storage/bufmgr.h"
#include "storage/io_worker.h"
#include "storage/ipc.h"
#include "storage/proc.h"
@@ -72,15 +73,9 @@ AioHandleShmemSize(void)
static Size
AioHandleIOVShmemSize(void)
{
/*
* Each IO handle can have an PG_IOV_MAX long iovec.
*
* XXX: Right now the amount of space available for each IO is PG_IOV_MAX.
* While it's tempting to use the io_combine_limit GUC, that's
* PGC_USERSET, so we can't allocate shared memory based on that.
*/
/* each IO handle can have up to io_max_combine_limit iovec objects */
return mul_size(sizeof(struct iovec),
mul_size(mul_size(PG_IOV_MAX, AioProcs()),
mul_size(mul_size(io_max_combine_limit, AioProcs()),
io_max_concurrency));
}
@@ -89,7 +84,7 @@ AioHandleDataShmemSize(void)
{
/* each buffer referenced by an iovec can have associated data */
return mul_size(sizeof(uint64),
mul_size(mul_size(PG_IOV_MAX, AioProcs()),
mul_size(mul_size(io_max_combine_limit, AioProcs()),
io_max_concurrency));
}
@@ -160,7 +155,7 @@ AioShmemInit(void)
bool found;
uint32 io_handle_off = 0;
uint32 iovec_off = 0;
uint32 per_backend_iovecs = io_max_concurrency * PG_IOV_MAX;
uint32 per_backend_iovecs = io_max_concurrency * io_max_combine_limit;
pgaio_ctl = (PgAioCtl *)
ShmemInitStruct("AioCtl", AioCtlShmemSize(), &found);
@@ -213,7 +208,7 @@ AioShmemInit(void)
ConditionVariableInit(&ioh->cv);
dclist_push_tail(&bs->idle_ios, &ioh->node);
iovec_off += PG_IOV_MAX;
iovec_off += io_max_combine_limit;
}
}

5
src/backend/storage/buffer/bufmgr.c
View File

@@ -160,9 +160,12 @@ int maintenance_io_concurrency = DEFAULT_MAINTENANCE_IO_CONCURRENCY;
/*
* Limit on how many blocks should be handled in single I/O operations.
* StartReadBuffers() callers should respect it, as should other operations
* that call smgr APIs directly.
* that call smgr APIs directly. It is computed as the minimum of underlying
* GUCs io_combine_limit_guc and io_max_combine_limit.
*/
int io_combine_limit = DEFAULT_IO_COMBINE_LIMIT;
int io_combine_limit_guc = DEFAULT_IO_COMBINE_LIMIT;
int io_max_combine_limit = DEFAULT_IO_COMBINE_LIMIT;
/*
* GUC variables about triggering kernel writeback for buffers written; OS