fix(l0_flush): drops permit before fsync, potential cause for OOMs #8327
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jcsp
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Jul 9, 2024
3042 tests run: 2927 passed, 0 failed, 115 skipped (full report)Flaky tests (3)Postgres 16
Postgres 15
Postgres 14
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* collected from Rust tests only The comment gets automatically updated with the latest test results
82057ca at 2024-07-09T17:18:37.651Z :recycle: |
skyzh
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Jul 15, 2024
…8327) ## Problem Slack thread: https://neondb.slack.com/archives/C033RQ5SPDH/p1720511577862519 We're seeing OOMs in staging on a pageserver that has l0_flush.mode=Direct enabled. There's a strong correlation between jumps in `maxrss_kb` and `pageserver_timeline_ephemeral_bytes`, so, it's quite likely that l0_flush.mode=Direct is the culprit. Notably, the expected max memory usage on that staging server by the l0_flush.mode=Direct is ~2GiB but we're seeing as much as 24GiB max RSS before the OOM kill. One hypothesis is that we're dropping the semaphore permit before all the dirtied pages have been flushed to disk. (The flushing to disk likely happens in the fsync inside the `.finish()` call, because we're using ext4 in data=ordered mode). ## Summary of changes Hold the permit until after we're done with `.finish()`.
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Problem
Slack thread: https://neondb.slack.com/archives/C033RQ5SPDH/p1720511577862519
We're seeing OOMs in staging on a pageserver that has l0_flush.mode=Direct enabled.
There's a strong correlation between jumps in
maxrss_kbandpageserver_timeline_ephemeral_bytes, so, it's quite likely that l0_flush.mode=Direct is the culprit.Notably, the expected max memory usage on that staging server by the l0_flush.mode=Direct is ~2GiB but we're seeing as much as 24GiB max RSS before the OOM kill.
One hypothesis is that we're dropping the semaphore permit before all the dirtied pages have been flushed to disk. (The flushing to disk likely happens in the fsync inside the
.finish()call, because we're using ext4 in data=ordered mode).Summary of changes
Hold the permit until after we're done with
.finish().