read_stream: Issue IO synchronously while in fast path

While in fast-path, execute any IO that we might encounter synchronously.
Because we are, in that moment, not reading ahead, dispatching any occasional
IO to workers has the dispatch overhead, without any realistic chance of the
IO completing before we need it.

This helps io_method=worker performance for workloads that have only
occasional cache misses, but where those occasional misses still take long
enough to matter.  It is likely this is only measurable with fast local
storage or workloads with the data in the kernel page cache, as with remote
storage the IO latency, not the dispatch-to-worker latency, is the determining
factor.

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu
Discussion: https://postgr.es/m/CAH2-Wz%3DkMg3PNay96cHMT0LFwtxP-cQSRZTZzh1Cixxf8G%3Dzrw%40mail.gmail.com

diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c
index cd54c1a74ac7270f9c08ebaab5440ddd35da4cf6..c9595ea10c7890b98b26dfb4c441f247a252bbc6 100644 (file)
--- a/src/backend/storage/aio/read_stream.c
+++ b/src/backend/storage/aio/read_stream.c
@@ -833,6 +833,21 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
                        if (stream->advice_enabled)
                                flags |= READ_BUFFERS_ISSUE_ADVICE;
 
+                       /*
+                        * While in fast-path, execute any IO that we might encounter
+                        * synchronously. Because we are, right now, only looking one
+                        * block ahead, dispatching any occasional IO to workers would
+                        * have the overhead of dispatching to workers, without any
+                        * realistic chance of the IO completing before we need it. We
+                        * will switch to non-synchronous IO after this.
+                        *
+                        * Arguably we should do so only for worker, as there's far less
+                        * dispatch overhead with io_uring. However, tests so far have not
+                        * shown a clear downside and additional io_method awareness here
+                        * seems not great from an abstraction POV.
+                        */
+                       flags |= READ_BUFFERS_SYNCHRONOUSLY;
+
                        /*
                         * Pin a buffer for the next call.  Same buffer entry, and
                         * arbitrary I/O entry (they're all free).  We don't have to
@@ -860,6 +875,12 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
                        stream->ios_in_progress = 1;
                        stream->ios[0].buffer_index = oldest_buffer_index;
                        stream->seq_blocknum = next_blocknum + 1;
+
+                       /*
+                        * XXX: It might be worth triggering additional read-ahead here,
+                        * to avoid having to effectively do another synchronous IO for
+                        * the next block (if it were also a miss).
+                        */
                }
                else
                {