in_tail: restrict file_cache_advise eviction to the consumed range - #12322
in_tail: restrict file_cache_advise eviction to the consumed range#12322baizhenyu wants to merge 1 commit into
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Testing evidence, as required by the template. Environment: Linux (aarch64), gcc 12 (Debian), branch built with Example configuration A writer appends ~1,000 records/s (~75 B each) to (equivalently: strace output showing the new advise pattern
The writer's in-progress page is never forced into writeback (no write amplification), unread data is never evicted, and because the consumed range is re-advised each cycle, pages that were still dirty on a previous call get evicted once their writeback completes (see the review discussion below). Valgrind
Debug log output (tail of the The disk-I/O impact numbers of the whole-file advise behavior (measured on GKE at identical log throughput) are in the PR description. |
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Included review availability: Your plan provides up to 8 included reviews per hour; 6 remain after this review. 📝 WalkthroughWalkthroughThe tail plugin replaces whole-file cache advice with Linux page-aligned advice for consumed data up to ChangesTail file cache advising
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🟡 Moderate · up to The change narrows cache eviction, but the advised range still follows the raw read position rather than the consumed position, so buffered data may be evicted before processing and later reread, undermining the intended behavior and adding I/O. Merge readiness remains moderate until this handling is corrected or explicitly accepted. Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@plugins/in_tail/tail_file.c`:
- Around line 2021-2024: Update the page-aligned POSIX_FADV_DONTNEED calculation
near page_size to use file->stream_offset for uncompressed input, rather than
file->offset, so only consumed raw bytes are advised; for compressed input, use
the decompressor’s consumed raw-input position or disable the advice.
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plugins/in_tail/tail_file.cplugins/in_tail/tail_file_internal.h
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Since fluent#8422, when file_cache_advise is enabled (the default on Linux), in_tail calls posix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED) over the whole file on every read cycle. This has two unintended side effects on files that are being actively appended to by a writer: 1. Write amplification: POSIX_FADV_DONTNEED starts writeback of dirty pages. The last page of an actively written log file is almost always dirty, so the same tail page is flushed to disk on every read cycle instead of once by the kernel flusher. 2. Cold reads for other consumers: the advice also evicts pages that other readers still need. For example, kubelet (Kubernetes) compresses rotated container log files; with the cache evicted it re-reads every rotated file from disk. Steady-state extra reads equal roughly 1x the log production rate. Measured on GKE (3 nodes, ~2.6 MB/s of logs per node, 10 min windows, whole-file advise vs no advise): disk reads 2.55 MB/s -> 0.00 MB/s, disk write IOPS 110 -> 76, at identical log throughput. The fix is to cap the advised range at the last page boundary below the consumed offset: posix_fadvise(fd, 0, aligned_end, POSIX_FADV_DONTNEED). The still-appended (dirty) tail page is never advised, so no page is flushed to disk more than once, and data that has not been read yet is never evicted. The advice is still issued on every read cycle so that pages which were dirty (and therefore not droppable) when first advised are evicted by a later call once their writeback completes; consumed pages are never re-dirtied by the writer, so the repetition causes no additional writes. This keeps the feature's purpose: consumed log data does not accumulate in the page cache. Also fix the error check: posix_fadvise() returns the error number and does not set errno, so the previous `== -1` check plus flb_errno() could never report a real failure. Signed-off-by: Tim Bai <timbai@google.com>
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Since #8422,
file_cache_advise(default on, Linux) runsposix_fadvise(fd, 0, 0, POSIX_FADV_DONTNEED)over the whole file on every read cycle. On actively written files this has two unintended side effects:POSIX_FADV_DONTNEEDstarts writeback of dirty pages. The tail page of an actively appended log file is almost always dirty, so the same page is flushed to disk on every read cycle instead of once by the kernel flusher.Measured on GKE (3 × n2-standard-8 nodes, ~2.6 MB/s of logs per node, whole-file advise vs no advise, identical log throughput):
This PR keeps the feature and its intent (consumed log data does not accumulate in the page cache) but caps the advised range at the last page boundary below the consumed offset:
posix_fadvise(fd, 0, aligned_end, POSIX_FADV_DONTNEED). The still-appended (dirty) tail page is never advised, so no page is flushed to disk more than once, and data not yet read by fluent-bit is never evicted. The advice is still issued on every read cycle, so pages that were dirty (and therefore not droppable) when first advised are evicted by a later call once their writeback completes — consumed pages are never re-dirtied by the writer, so the repetition causes no additional writes. Per-file cache footprint stays bounded at roughly the trailing writeback window plus unconsumed data.Also fixes the error check:
posix_fadvise()returns the error number and does not seterrno, so the previous== -1+flb_errno()check could never report a real failure.Scope note: this change removes the write amplification and the eviction of not-yet-read data, but it intentionally keeps the feature's core semantics: consumed data is still dropped from the page cache. Deployments where another process re-reads the log files afterwards (e.g. kubelet compressing rotated container logs on Kubernetes) will still see those readers go to disk, and should set
file_cache_advise false. Given the measurements above, it may also be worth discussing whether the option should remain enabled by default; that question is left out of this PR.Enter
[N/A]in the box, if an item is not applicable to your change.Testing
Before we can approve your change; please submit the following in a comment:
If this is a change to packaging of containers or native binaries then please confirm it works for all targets.
[N/A]Run local packaging test showing all targets (including any new ones) build.[N/A]Setok-package-testlabel to test for all targets (requires maintainer to do).Documentation
[N/A]Documentation required for this feature (no configuration surface change;file_cache_advisesemantics are preserved, only the advised range is narrowed)Backporting
Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.
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