Re: index prefetching

On Mon Aug 25, 2025 at 10:18 AM EDT, Tomas Vondra wrote:
> The attached patch is a PoC implementing this. The core idea is that if
> we measure "miss probability" for a chunk of requests, we can use that
> to estimate the distance needed to generate e_i_c IOs.

I noticed an assertion failure when the tests run. Looks like something about
the patch breaks the read stream from the point of view of VACUUM:

TRAP: failed Assert("stream->pinned_buffers + stream->pending_read_nblocks <= stream->max_pinned_buffers"), File:
"../source/src/backend/storage/aio/read_stream.c",Line: 402, PID: 1238204 
[0x55e71f653d29] read_stream_start_pending_read:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/storage/aio/read_stream.c:401
[0x55e71f6533ad] read_stream_look_ahead:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/storage/aio/read_stream.c:670
[0x55e71f652e9a] read_stream_next_buffer:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/storage/aio/read_stream.c:1173
[0x55e71f34cd2b] lazy_scan_heap:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/access/heap/vacuumlazy.c:1310
[0x55e71f34cd2b] heap_vacuum_rel:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/access/heap/vacuumlazy.c:839
[0x55e71f49a3f4] table_relation_vacuum: ../source/src/include/access/tableam.h:1670
[0x55e71f49a3f4] vacuum_rel: /mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/commands/vacuum.c:2296
[0x55e71f499e8f] vacuum: /mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/commands/vacuum.c:636
[0x55e71f49931d] ExecVacuum: /mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/commands/vacuum.c:468
[0x55e71f6a69f7] standard_ProcessUtility:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/tcop/utility.c:862
[0x55e71f6a67d7] ProcessUtility: /mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/tcop/utility.c:523
[0x55e71f6a630b] PortalRunUtility: /mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/tcop/pquery.c:1153
[0x55e71f6a59b3] PortalRunMulti: /mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/tcop/pquery.c:0
[0x55e71f6a52c5] PortalRun: /mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/tcop/pquery.c:788
[0x55e71f6a4119] exec_simple_query:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/tcop/postgres.c:1274
[0x55e71f6a1b84] PostgresMain: /mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/tcop/postgres.c:0
[0x55e71f69c078] BackendMain:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/tcop/backend_startup.c:124
[0x55e71f5e5eda] postmaster_child_launch:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/postmaster/launch_backend.c:290
[0x55e71f5ea847] BackendStartup:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/postmaster/postmaster.c:3587
[0x55e71f5ea847] ServerLoop:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/postmaster/postmaster.c:1702
[0x55e71f5e86d9] PostmasterMain:
/mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/postmaster/postmaster.c:1400
[0x55e71f51acd9] main: /mnt/nvme/postgresql/patch/build_meson_dc/../source/src/backend/main/main.c:231
[0x7ff312633ca7] __libc_start_call_main: ../sysdeps/nptl/libc_start_call_main.h:58
[0x7ff312633d64] __libc_start_main_impl: ../csu/libc-start.c:360
[0x55e71f2e09a0] [unknown]: [unknown]:0

2025-08-25 21:05:28.915 EDT postmaster[1236725] LOG:  client backend (PID 1238204) was terminated by signal 6: Aborted
2025-08-25 21:05:28.915 EDT postmaster[1236725] DETAIL:  Failed process was running: VACUUM (PARALLEL 0,
BUFFER_USAGE_LIMIT128) test_io_vac_strategy; 
2025-08-25 21:05:28.915 EDT postmaster[1236725] LOG:  terminating any other active server processes
2025-08-25 21:05:28.915 EDT postmaster[1236725] LOG:  all server processes terminated; reinitializing

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Peter Geoghegan