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readahead: clean up the code, update docs

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Remove dubious comments and #if 0 section.  Document new event counters,
and add one for read failures.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>
This commit is contained in:
Zygo Blaxell 2025-01-22 22:28:39 -05:00
parent 48b7fbda9c
commit 30cd375d03
2 changed files with 12 additions and 10 deletions
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7
docs/event-counters.md
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@ -281,11 +281,14 @@ The `progress` event group consists of events related to progress estimation.
readahead
---------
The `readahead` event group consists of events related to calls to `posix_fadvise`.
The `readahead` event group consists of events related to data prefetching (formerly calls to `posix_fadvise` or `readahead`, but now emulated in userspace).
* `readahead_bytes`: Number of bytes prefetched.
* `readahead_count`: Number of read calls.
* `readahead_clear`: Number of times the duplicate read cache was cleared.
* `readahead_skip`: Number of times a duplicate read was identified in the cache and skipped.
* `readahead_fail`: Number of read errors during prefetch.
* `readahead_ms`: Total time spent emulating readahead in user-space (kernel readahead is not measured).
* `readahead_skip`: Number of times a duplicate read was identified in the cache and skipped.
* `readahead_unread_ms`: Total time spent running `posix_fadvise(..., POSIX_FADV_DONTNEED)`.
replacedst

15
src/bees.cc
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@ -246,10 +246,6 @@ bees_readahead_nolock(int const fd, const off_t offset, const size_t size)
Timer readahead_timer;
BEESNOTE("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
BEESTOOLONG("readahead " << name_fd(fd) << " offset " << to_hex(offset) << " len " << pretty(size));
#if 0
// In the kernel, readahead() is identical to posix_fadvise(..., POSIX_FADV_DONTNEED)
DIE_IF_NON_ZERO(readahead(fd, offset, size));
#else
// Make sure this data is in page cache by brute force
// The btrfs kernel code does readahead with lower ioprio
// and might discard the readahead request entirely.
@ -263,13 +259,16 @@ bees_readahead_nolock(int const fd, const off_t offset, const size_t size)
// Ignore errors and short reads. It turns out our size
// parameter isn't all that accurate, so we can't use
// the pread_or_die template.
(void)!pread(fd, dummy, this_read_size, working_offset);
BEESCOUNT(readahead_count);
BEESCOUNTADD(readahead_bytes, this_read_size);
const auto pr_rv = pread(fd, dummy, this_read_size, working_offset);
if (pr_rv >= 0) {
BEESCOUNT(readahead_count);
BEESCOUNTADD(readahead_bytes, pr_rv);
} else {
BEESCOUNT(readahead_fail);
}
working_offset += this_read_size;
working_size -= this_read_size;
}
#endif
BEESCOUNTADD(readahead_ms, readahead_timer.age() * 1000);
}