On Sun, Apr 07, 2024 at 03:00:00PM -0700, Andres Freund wrote:
> Hi,
>
> On 2024-04-07 16:59:26 -0400, Melanie Plageman wrote:
> > From 1dc2343661f3edb3b1bc4307afb0e956397eb76c Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <[email protected]>
> > Date: Sun, 7 Apr 2024 14:55:22 -0400
> > Subject: [PATCH v10 1/3] Make heapam_scan_analyze_next_[tuple|block] static.
> >
> > 27bc1772fc81 removed the table AM callbacks scan_analyze_next_block and
> > scan_analzye_next_tuple -- leaving their heap AM implementations only
> > called by acquire_sample_rows().
>
> Ugh, I don't think 27bc1772fc81 makes much sense. But that's unrelated to this
> thread. I did raise that separately
> https://www.postgresql.org/message-id/20240407214001.jgpg5q3yv33ve6y3%40awork3.anarazel.de
>
> Unless I seriously missed something, I see no alternative to reverting that
> commit.
Noted. I'll give up on this refactor then. Lots of churn for no gain.
Attached v11 is just Bilal's v8 patch rebased to apply cleanly and with
a few tweaks (I changed one of the loop conditions. All other changes
are to comments and commit message).
> > @@ -1206,11 +1357,13 @@ acquire_sample_rows(Relation onerel, int elevel,
> > break;
> >
> > prefetch_block = BlockSampler_Next(&prefetch_bs);
> > - PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM,
> > prefetch_block);
> > + PrefetchBuffer(scan->rs_base.rs_rd, MAIN_FORKNUM,
> > prefetch_block);
> > }
> > }
> > #endif
> >
> > + scan->rs_cbuf = InvalidBuffer;
> > +
> > /* Outer loop over blocks to sample */
> > while (BlockSampler_HasMore(&bs))
> > {
>
> I don't think it's good to move a lot of code *and* change how it is
> structured in the same commit. Makes it much harder to actually see changes /
> makes git blame harder to use / etc.
Yep.
> > From 90d115c2401567be65bcf64393a6d3b39286779e Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <[email protected]>
> > Date: Sun, 7 Apr 2024 15:28:32 -0400
> > Subject: [PATCH v10 2/3] Use streaming read API in ANALYZE
> >
> > The ANALYZE command prefetches and reads sample blocks chosen by a
> > BlockSampler algorithm. Instead of calling Prefetch|ReadBuffer() for
> > each block, ANALYZE now uses the streaming API introduced in b5a9b18cd0.
> >
> > Author: Nazir Bilal Yavuz
> > Reviewed-by: Melanie Plageman
> > Discussion:
> > https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com
> > ---
> > src/backend/commands/analyze.c | 89 ++++++++++------------------------
> > 1 file changed, 26 insertions(+), 63 deletions(-)
>
> That's a very nice demonstration of how this makes good prefetching easier...
Agreed. Yay streaming read API and Bilal!
> > From 862b7ac81cdafcda7b525e02721da14e46265509 Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <[email protected]>
> > Date: Sun, 7 Apr 2024 15:38:41 -0400
> > Subject: [PATCH v10 3/3] Obsolete BlockSampler_HasMore()
> >
> > A previous commit stopped using BlockSampler_HasMore() for flow control
> > in acquire_sample_rows(). There seems little use now for
> > BlockSampler_HasMore(). It should be sufficient to return
> > InvalidBlockNumber from BlockSampler_Next() when BlockSample_HasMore()
> > would have returned false. Remove BlockSampler_HasMore().
> >
> > Author: Melanie Plageman, Nazir Bilal Yavuz
> > Discussion:
> > https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com
>
> The justification here seems somewhat odd. Sure, the previous commit stopped
> using BlockSampler_HasMore in acquire_sample_rows - but only because it was
> moved to block_sampling_streaming_read_next()?
It didn't stop using it. It stopped being useful. The reason it existed,
as far as I can tell, was to use it as the while() loop condition in
acquire_sample_rows(). I think it makes much more sense for
BlockSampler_Next() to return InvalidBlockNumber when there are no more
blocks -- not to assert you don't call it when there aren't any more
blocks.
I didn't want to change BlockSampler_Next() in the same commit as the
streaming read user and we can't remove BlockSampler_HasMore() without
changing BlockSampler_Next().
- Melanie
>From 3cb43693c04554f5d46e0dc9156bef36af642593 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <[email protected]>
Date: Sun, 7 Apr 2024 18:17:01 -0400
Subject: [PATCH v11 1/2] Use streaming read API in ANALYZE
The ANALYZE command prefetches and reads sample blocks chosen by a
BlockSampler algorithm. Instead of calling [Prefetch|Read]Buffer() for
each block, ANALYZE now uses the streaming API introduced in b5a9b18cd0.
Author: Nazir Bilal Yavuz
Reviewed-by: Melanie Plageman, Andres Freund
Discussion: https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com
---
src/backend/access/heap/heapam_handler.c | 20 +++---
src/backend/commands/analyze.c | 85 +++++++-----------------
src/include/access/heapam.h | 5 +-
3 files changed, 39 insertions(+), 71 deletions(-)
diff --git a/src/backend/access/heap/heapam_handler.c b/src/backend/access/heap/heapam_handler.c
index 58de2c82a70..d9f053022f2 100644
--- a/src/backend/access/heap/heapam_handler.c
+++ b/src/backend/access/heap/heapam_handler.c
@@ -1055,16 +1055,16 @@ heapam_relation_copy_for_cluster(Relation OldHeap, Relation NewHeap,
}
/*
- * Prepare to analyze block `blockno` of `scan`. The scan has been started
+ * Prepare to analyze the next block in the a read stream. Returns false if
+ * the stream is exhausted and true otherwise. The scan must have been started
* with SO_TYPE_ANALYZE option.
*
* This routine holds a buffer pin and lock on the heap page. They are held
* until heapam_scan_analyze_next_tuple() returns false. That is until all the
* items of the heap page are analyzed.
*/
-void
-heapam_scan_analyze_next_block(TableScanDesc scan, BlockNumber blockno,
- BufferAccessStrategy bstrategy)
+bool
+heapam_scan_analyze_next_block(TableScanDesc scan, ReadStream *stream)
{
HeapScanDesc hscan = (HeapScanDesc) scan;
@@ -1077,11 +1077,15 @@ heapam_scan_analyze_next_block(TableScanDesc scan, BlockNumber blockno,
* doing much work per tuple, the extra lock traffic is probably better
* avoided.
*/
- hscan->rs_cblock = blockno;
- hscan->rs_cindex = FirstOffsetNumber;
- hscan->rs_cbuf = ReadBufferExtended(scan->rs_rd, MAIN_FORKNUM,
- blockno, RBM_NORMAL, bstrategy);
+ hscan->rs_cbuf = read_stream_next_buffer(stream, NULL);
+ if (!BufferIsValid(hscan->rs_cbuf))
+ return false;
+
LockBuffer(hscan->rs_cbuf, BUFFER_LOCK_SHARE);
+
+ hscan->rs_cblock = BufferGetBlockNumber(hscan->rs_cbuf);
+ hscan->rs_cindex = FirstOffsetNumber;
+ return true;
}
/*
diff --git a/src/backend/commands/analyze.c b/src/backend/commands/analyze.c
index 2fb39f3ede1..58f6ad96136 100644
--- a/src/backend/commands/analyze.c
+++ b/src/backend/commands/analyze.c
@@ -1102,6 +1102,20 @@ examine_attribute(Relation onerel, int attnum, Node *index_expr)
return stats;
}
+/*
+ * Streaming read callback returning the next BlockNumber as chosen by the
+ * BlockSampling algorithm.
+ */
+static BlockNumber
+block_sampling_streaming_read_next(ReadStream *stream,
+ void *user_data,
+ void *per_buffer_data)
+{
+ BlockSamplerData *bs = user_data;
+
+ return BlockSampler_HasMore(bs) ? BlockSampler_Next(bs) : InvalidBlockNumber;
+}
+
/*
* acquire_sample_rows -- acquire a random sample of rows from the heap
*
@@ -1154,10 +1168,7 @@ acquire_sample_rows(Relation onerel, int elevel,
TableScanDesc scan;
BlockNumber nblocks;
BlockNumber blksdone = 0;
-#ifdef USE_PREFETCH
- int prefetch_maximum = 0; /* blocks to prefetch if enabled */
- BlockSamplerData prefetch_bs;
-#endif
+ ReadStream *stream;
Assert(targrows > 0);
@@ -1170,13 +1181,6 @@ acquire_sample_rows(Relation onerel, int elevel,
randseed = pg_prng_uint32(&pg_global_prng_state);
nblocks = BlockSampler_Init(&bs, totalblocks, targrows, randseed);
-#ifdef USE_PREFETCH
- prefetch_maximum = get_tablespace_maintenance_io_concurrency(onerel->rd_rel->reltablespace);
- /* Create another BlockSampler, using the same seed, for prefetching */
- if (prefetch_maximum)
- (void) BlockSampler_Init(&prefetch_bs, totalblocks, targrows, randseed);
-#endif
-
/* Report sampling block numbers */
pgstat_progress_update_param(PROGRESS_ANALYZE_BLOCKS_TOTAL,
nblocks);
@@ -1187,60 +1191,19 @@ acquire_sample_rows(Relation onerel, int elevel,
scan = heap_beginscan(onerel, NULL, 0, NULL, NULL, SO_TYPE_ANALYZE);
slot = table_slot_create(onerel, NULL);
-#ifdef USE_PREFETCH
-
- /*
- * If we are doing prefetching, then go ahead and tell the kernel about
- * the first set of pages we are going to want. This also moves our
- * iterator out ahead of the main one being used, where we will keep it so
- * that we're always pre-fetching out prefetch_maximum number of blocks
- * ahead.
- */
- if (prefetch_maximum)
- {
- for (int i = 0; i < prefetch_maximum; i++)
- {
- BlockNumber prefetch_block;
-
- if (!BlockSampler_HasMore(&prefetch_bs))
- break;
-
- prefetch_block = BlockSampler_Next(&prefetch_bs);
- PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, prefetch_block);
- }
- }
-#endif
+ stream = read_stream_begin_relation(READ_STREAM_MAINTENANCE,
+ vac_strategy,
+ scan->rs_rd,
+ MAIN_FORKNUM,
+ block_sampling_streaming_read_next,
+ &bs,
+ 0);
/* Outer loop over blocks to sample */
- while (BlockSampler_HasMore(&bs))
+ while (heapam_scan_analyze_next_block(scan, stream))
{
- BlockNumber targblock = BlockSampler_Next(&bs);
-#ifdef USE_PREFETCH
- BlockNumber prefetch_targblock = InvalidBlockNumber;
-
- /*
- * Make sure that every time the main BlockSampler is moved forward
- * that our prefetch BlockSampler also gets moved forward, so that we
- * always stay out ahead.
- */
- if (prefetch_maximum && BlockSampler_HasMore(&prefetch_bs))
- prefetch_targblock = BlockSampler_Next(&prefetch_bs);
-#endif
-
vacuum_delay_point();
- heapam_scan_analyze_next_block(scan, targblock, vac_strategy);
-
-#ifdef USE_PREFETCH
-
- /*
- * When pre-fetching, after we get a block, tell the kernel about the
- * next one we will want, if there's any left.
- */
- if (prefetch_maximum && prefetch_targblock != InvalidBlockNumber)
- PrefetchBuffer(scan->rs_rd, MAIN_FORKNUM, prefetch_targblock);
-#endif
-
while (heapam_scan_analyze_next_tuple(scan, OldestXmin, &liverows, &deadrows, slot))
{
/*
@@ -1290,6 +1253,8 @@ acquire_sample_rows(Relation onerel, int elevel,
++blksdone);
}
+ read_stream_end(stream);
+
ExecDropSingleTupleTableSlot(slot);
heap_endscan(scan);
diff --git a/src/include/access/heapam.h b/src/include/access/heapam.h
index 48936826bcc..f84dbe629fe 100644
--- a/src/include/access/heapam.h
+++ b/src/include/access/heapam.h
@@ -413,9 +413,8 @@ extern bool HeapTupleIsSurelyDead(HeapTuple htup,
struct GlobalVisState *vistest);
/* in heap/heapam_handler.c*/
-extern void heapam_scan_analyze_next_block(TableScanDesc scan,
- BlockNumber blockno,
- BufferAccessStrategy bstrategy);
+extern bool heapam_scan_analyze_next_block(TableScanDesc scan,
+ ReadStream *stream);
extern bool heapam_scan_analyze_next_tuple(TableScanDesc scan,
TransactionId OldestXmin,
double *liverows, double *deadrows,
--
2.40.1
>From e7de413aedc33a77f9758fd37872f2eb2b6478f5 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <[email protected]>
Date: Sun, 7 Apr 2024 18:19:38 -0400
Subject: [PATCH v11 2/2] Obsolete BlockSampler_HasMore()
A previous commit stopped using BlockSampler_HasMore() for flow control
in acquire_sample_rows(). There seems little use now for
BlockSampler_HasMore(). It should be sufficient to return
InvalidBlockNumber from BlockSampler_Next() when BlockSample_HasMore()
would have returned false. Remove BlockSampler_HasMore().
Author: Melanie Plageman, Nazir Bilal Yavuz
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/flat/CAN55FZ0UhXqk9v3y-zW_fp4-WCp43V8y0A72xPmLkOM%2B6M%2BmJg%40mail.gmail.com
---
src/backend/commands/analyze.c | 4 +---
src/backend/utils/misc/sampling.c | 10 +++-------
src/include/utils/sampling.h | 1 -
3 files changed, 4 insertions(+), 11 deletions(-)
diff --git a/src/backend/commands/analyze.c b/src/backend/commands/analyze.c
index 58f6ad96136..602d0f35e7c 100644
--- a/src/backend/commands/analyze.c
+++ b/src/backend/commands/analyze.c
@@ -1111,9 +1111,7 @@ block_sampling_streaming_read_next(ReadStream *stream,
void *user_data,
void *per_buffer_data)
{
- BlockSamplerData *bs = user_data;
-
- return BlockSampler_HasMore(bs) ? BlockSampler_Next(bs) : InvalidBlockNumber;
+ return BlockSampler_Next(user_data);
}
/*
diff --git a/src/backend/utils/misc/sampling.c b/src/backend/utils/misc/sampling.c
index 933db06702c..6e2bca9739c 100644
--- a/src/backend/utils/misc/sampling.c
+++ b/src/backend/utils/misc/sampling.c
@@ -54,12 +54,6 @@ BlockSampler_Init(BlockSampler bs, BlockNumber nblocks, int samplesize,
return Min(bs->n, bs->N);
}
-bool
-BlockSampler_HasMore(BlockSampler bs)
-{
- return (bs->t < bs->N) && (bs->m < bs->n);
-}
-
BlockNumber
BlockSampler_Next(BlockSampler bs)
{
@@ -68,7 +62,9 @@ BlockSampler_Next(BlockSampler bs)
double p; /* probability to skip block */
double V; /* random */
- Assert(BlockSampler_HasMore(bs)); /* hence K > 0 and k > 0 */
+ /* Return if no remaining blocks or no blocks to sample */
+ if (K <= 0 || k <= 0)
+ return InvalidBlockNumber;
if ((BlockNumber) k >= K)
{
diff --git a/src/include/utils/sampling.h b/src/include/utils/sampling.h
index be48ee52bac..fb5d6820a24 100644
--- a/src/include/utils/sampling.h
+++ b/src/include/utils/sampling.h
@@ -38,7 +38,6 @@ typedef BlockSamplerData *BlockSampler;
extern BlockNumber BlockSampler_Init(BlockSampler bs, BlockNumber nblocks,
int samplesize, uint32 randseed);
-extern bool BlockSampler_HasMore(BlockSampler bs);
extern BlockNumber BlockSampler_Next(BlockSampler bs);
/* Reservoir sampling methods */
--
2.40.1
