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Fix read_stream.c for changing io_combine_limit.

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In a couple of places, read_stream.c assumed that io_combine_limit would
be stable during the lifetime of a stream.  That is not true in at least
one unusual case: streams held by CURSORs where you could change the GUC
between FETCH commands, with unpredictable results.

Fix, by storing stream->io_combine_limit and referring only to that
after construction.  This mirrors the treatment of the other important
setting {effective,maintenance}_io_concurrency, which is stored in
stream->max_ios.

One of the cases was the queue overflow space, which was sized for
io_combine_limit and could be overrun if the GUC was increased.  Since
that coding was a little hard to follow, also introduce a variable for
better readability instead of open-coding the arithmetic.  Doing so
revealed an off-by-one thinko while clamping max_pinned_buffers to
INT16_MAX, though that wasn't a live bug due to the current limits on
GUC values.

Back-patch to 17.

Discussion: https://postgr.es/m/CA%2BhUKG%2B2T9p-%2BzM6Eeou-RAJjTML6eit1qn26f9twznX59qtCA%40mail.gmail.com
This commit is contained in:
Thomas Munro 2025-03-13 15:43:34 +13:00
parent 8b2392ae3d
commit e273468070
1 changed files with 27 additions and 11 deletions
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38
src/backend/storage/aio/read_stream.c
View File

@ -109,6 +109,7 @@ typedef struct InProgressIO
struct ReadStream
{
int16 max_ios;
int16 io_combine_limit;
int16 ios_in_progress;
int16 queue_size;
int16 max_pinned_buffers;
@ -219,7 +220,7 @@ read_stream_start_pending_read(ReadStream *stream, bool suppress_advice)
/* This should only be called with a pending read. */
Assert(stream->pending_read_nblocks > 0);
Assert(stream->pending_read_nblocks <= io_combine_limit);
Assert(stream->pending_read_nblocks <= stream->io_combine_limit);
/* We had better not exceed the pin limit by starting this read. */
Assert(stream->pinned_buffers + stream->pending_read_nblocks <=
@ -307,7 +308,7 @@ read_stream_look_ahead(ReadStream *stream, bool suppress_advice)
int16 buffer_index;
void *per_buffer_data;
if (stream->pending_read_nblocks == io_combine_limit)
if (stream->pending_read_nblocks == stream->io_combine_limit)
{
read_stream_start_pending_read(stream, suppress_advice);
suppress_advice = false;
@ -367,7 +368,7 @@ read_stream_look_ahead(ReadStream *stream, bool suppress_advice)
* signaled end-of-stream, we start the read immediately.
*/
if (stream->pending_read_nblocks > 0 &&
(stream->pending_read_nblocks == io_combine_limit ||
(stream->pending_read_nblocks == stream->io_combine_limit ||
(stream->pending_read_nblocks == stream->distance &&
stream->pinned_buffers == 0) ||
stream->distance == 0) &&
@ -396,6 +397,7 @@ read_stream_begin_relation(int flags,
ReadStream *stream;
size_t size;
int16 queue_size;
int16 queue_overflow;
int max_ios;
int strategy_pin_limit;
uint32 max_pinned_buffers;
@ -429,6 +431,14 @@ read_stream_begin_relation(int flags,
/* Cap to INT16_MAX to avoid overflowing below */
max_ios = Min(max_ios, PG_INT16_MAX);
/*
* If starting a multi-block I/O near the end of the queue, we might
* temporarily need extra space for overflowing buffers before they are
* moved to regular circular position. This is the maximum extra space we
* could need.
*/
queue_overflow = io_combine_limit - 1;
/*
* Choose the maximum number of buffers we're prepared to pin. We try to
* pin fewer if we can, though. We clamp it to at least io_combine_limit
@ -439,7 +449,7 @@ read_stream_begin_relation(int flags,
*/
max_pinned_buffers = Max(max_ios * 4, io_combine_limit);
max_pinned_buffers = Min(max_pinned_buffers,
PG_INT16_MAX - io_combine_limit - 1);
PG_INT16_MAX - queue_overflow - 1);
/* Give the strategy a chance to limit the number of buffers we pin. */
strategy_pin_limit = GetAccessStrategyPinLimit(strategy);
@ -465,18 +475,17 @@ read_stream_begin_relation(int flags,
* one big chunk. Though we have queue_size buffers, we want to be able
* to assume that all the buffers for a single read are contiguous (i.e.
* don't wrap around halfway through), so we allow temporary overflows of
* up to the maximum possible read size by allocating an extra
* io_combine_limit - 1 elements.
* up to the maximum possible overflow size.
*/
size = offsetof(ReadStream, buffers);
size += sizeof(Buffer) * (queue_size + io_combine_limit - 1);
size += sizeof(Buffer) * (queue_size + queue_overflow);
size += sizeof(InProgressIO) * Max(1, max_ios);
size += per_buffer_data_size * queue_size;
size += MAXIMUM_ALIGNOF * 2;
stream = (ReadStream *) palloc(size);
memset(stream, 0, offsetof(ReadStream, buffers));
stream->ios = (InProgressIO *)
MAXALIGN(&stream->buffers[queue_size + io_combine_limit - 1]);
MAXALIGN(&stream->buffers[queue_size + queue_overflow]);
if (per_buffer_data_size > 0)
stream->per_buffer_data = (void *)
MAXALIGN(&stream->ios[Max(1, max_ios)]);
@ -503,7 +512,14 @@ read_stream_begin_relation(int flags,
if (max_ios == 0)
max_ios = 1;
/*
* Capture stable values for these two GUC-derived numbers for the
* lifetime of this stream, so we don't have to worry about the GUCs
* changing underneath us beyond this point.
*/
stream->max_ios = max_ios;
stream->io_combine_limit = io_combine_limit;
stream->per_buffer_data_size = per_buffer_data_size;
stream->max_pinned_buffers = max_pinned_buffers;
stream->queue_size = queue_size;
@ -517,7 +533,7 @@ read_stream_begin_relation(int flags,
* doing full io_combine_limit sized reads (behavior B).
*/
if (flags & READ_STREAM_FULL)
stream->distance = Min(max_pinned_buffers, io_combine_limit);
stream->distance = Min(max_pinned_buffers, stream->io_combine_limit);
else
stream->distance = 1;
@ -683,14 +699,14 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
else
{
/* No advice; move towards io_combine_limit (behavior B). */
if (stream->distance > io_combine_limit)
if (stream->distance > stream->io_combine_limit)
{
stream->distance--;
}
else
{
distance = stream->distance * 2;
distance = Min(distance, io_combine_limit);
distance = Min(distance, stream->io_combine_limit);
distance = Min(distance, stream->max_pinned_buffers);
stream->distance = distance;
}