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eviction: remove EvictionStats::not_considered_due_to_clock_skew

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Rationale: see the block comment added in this patch.

fixes #3641
This commit is contained in:
Christian Schwarz
2023-02-20 16:29:39 +01:00
committed by Christian Schwarz
parent e363911c85
commit ee1eda9921
1 changed files with 16 additions and 6 deletions
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22
pageserver/src/tenant/timeline/eviction_task.rs
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@@ -100,7 +100,6 @@ impl Timeline {
#[allow(dead_code)]
#[derive(Debug, Default)]
struct EvictionStats {
not_considered_due_to_clock_skew: usize,
candidates: usize,
evicted: usize,
errors: usize,
@@ -129,9 +128,21 @@ impl Timeline {
let no_activity_for = match now.duration_since(last_activity_ts) {
Ok(d) => d,
Err(_e) => {
// NB: don't log the error. If there are many layers and the system clock
// is skewed, we'd be flooding the log.
stats.not_considered_due_to_clock_skew += 1;
// We reach here if `now` < `last_activity_ts`, which can legitimately
// happen if there is an access between us getting `now`, and us getting
// the access stats from the layer.
//
// The other reason why it can happen is system clock skew because
// SystemTime::now() is not monotonic, so, even if there is no access
// to the layer after we get `now` at the beginning of this function,
// it could be that `now` < `last_activity_ts`.
//
// To distinguish the cases, we would need to record `Instant`s in the
// access stats (i.e., monotonic timestamps), but then, the timestamps
// values in the access stats would need to be `Instant`'s, and hence
// they would be meaningless outside of the pageserver process.
// At the time of writing, the trade-off is that access stats are more
// valuable than detecting clock skew.
continue;
}
};
@@ -188,8 +199,7 @@ impl Timeline {
}
}
}
if stats.not_considered_due_to_clock_skew > 0 || stats.errors > 0 || stats.not_evictable > 0
{
if stats.errors > 0 || stats.not_evictable > 0 {
warn!(stats=?stats, "eviction iteration complete");
} else {
info!(stats=?stats, "eviction iteration complete");