From 74b45e6a6387f0b7a0f12060a0d8cb401a85552e Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Sat, 15 Jun 2024 14:37:26 +1200 Subject: [PATCH] Remember ReadStream look-ahead distance on reset. Some ReadStream clients need to do more work to make a new stream of block numbers available. The callback indicates that block numbers have run out by returning InvalidBlockNumber, but some time later the client code resets the ReadStream to tell it that the callback now has more block number, so that it can begin consuming buffers again. For example, pgvector's HNSW index scan reach a new page, and find a new set of connections to many other pages that will soon be accesed. When the stream is reset, it would previously reset its look-ahead distance to 1, and have to build it back up again if I/O is necessary. With this patch, it will remember what it had before, which seems like a better bet. If that's wrong, it will soon ramp down via the usual algorithm. Discussion: https://postgr.es/m/CA%2BhUKGJ_7NKd46nx1wbyXWriuZSNzsTfm%2BrhEuvU6nxZi3-KVw%40mail.gmail.com --- src/backend/storage/aio/read_stream.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c index 7f0e07d9586..5d3e070afae 100644 --- a/src/backend/storage/aio/read_stream.c +++ b/src/backend/storage/aio/read_stream.c @@ -115,6 +115,7 @@ struct ReadStream int16 max_pinned_buffers; int16 pinned_buffers; int16 distance; + int16 reset_distance; bool advice_enabled; /* @@ -335,6 +336,7 @@ read_stream_look_ahead(ReadStream *stream, bool suppress_advice) if (blocknum == InvalidBlockNumber) { /* End of stream. */ + stream->reset_distance = stream->distance; stream->distance = 0; break; } @@ -526,6 +528,7 @@ read_stream_begin_impl(int flags, stream->distance = Min(max_pinned_buffers, io_combine_limit); else stream->distance = 1; + stream->reset_distance = stream->distance; /* * Since we always access the same relation, we can initialize parts of @@ -822,8 +825,11 @@ read_stream_reset(ReadStream *stream) Assert(stream->pinned_buffers == 0); Assert(stream->ios_in_progress == 0); - /* Start off assuming data is cached. */ - stream->distance = 1; + /* + * If the callback ran out of blocks temporarily, restore the distance from + * before. + */ + stream->distance = Max(stream->reset_distance, 1); } /* -- 2.46.0