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Avoid calling find_lsn_for_timestamp call while holding lock.

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Refactor update_gc_info function so that it calls the potentially
expensive find_lsn_for_timestamp() function before acquiring the
lock. This will also be needed if we make find_lsn_for_timestamp()
async in the future; it cannot be awaited while holding the lock.
This commit is contained in:
Heikki Linnakangas
2022-12-28 10:48:27 +02:00
committed by Heikki Linnakangas
parent 434fcac357
commit fefe19a284
1 changed files with 41 additions and 26 deletions
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67
pageserver/src/tenant/timeline.rs
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@@ -2674,29 +2674,27 @@ impl Timeline {
///
/// The 'pitr' duration is used to calculate a 'pitr_cutoff', which can be used to determine
/// whether a record is needed for PITR.
///
/// NOTE: This function holds a short-lived lock to protect the 'gc_info'
/// field, so that the three values passed as argument are stored
/// atomically. But the caller is responsible for ensuring that no new
/// branches are created that would need to be included in 'retain_lsns',
/// for example. The caller should hold `Tenant::gc_cs` lock to ensure
/// that.
///
pub(super) async fn update_gc_info(
&self,
retain_lsns: Vec<Lsn>,
cutoff_horizon: Lsn,
pitr: Duration,
) -> anyhow::Result<()> {
let mut gc_info = self.gc_info.write().unwrap();
gc_info.horizon_cutoff = cutoff_horizon;
gc_info.retain_lsns = retain_lsns;
// Calculate pitr cutoff point.
// If we cannot determine a cutoff LSN, be conservative and don't GC anything.
let mut pitr_cutoff_lsn: Lsn;
if pitr != Duration::ZERO {
// conservative, safe default is to remove nothing, when we have no
// commit timestamp data available
pitr_cutoff_lsn = *self.get_latest_gc_cutoff_lsn();
// First, calculate pitr_cutoff_timestamp and then convert it to LSN.
// If we don't have enough data to convert to LSN,
// play safe and don't remove any layers.
// First, calculate pitr_cutoff_timestamp and then convert it to LSN.
//
// Some unit tests depend on garbage-collection working even when
// CLOG data is missing, so that find_lsn_for_timestamp() doesn't
// work, so avoid calling it altogether if time-based retention is not
// configured. It would be pointless anyway.
let pitr_cutoff = if pitr != Duration::ZERO {
let now = SystemTime::now();
if let Some(pitr_cutoff_timestamp) = now.checked_sub(pitr) {
let pitr_timestamp = to_pg_timestamp(pitr_cutoff_timestamp);
@@ -2705,27 +2703,44 @@ impl Timeline {
.find_lsn_for_timestamp(pitr_timestamp)
.no_ondemand_download()?
{
LsnForTimestamp::Present(lsn) => pitr_cutoff_lsn = lsn,
LsnForTimestamp::Present(lsn) => lsn,
LsnForTimestamp::Future(lsn) => {
// The timestamp is in the future. That sounds impossible,
// but what it really means is that there hasn't been
// any commits since the cutoff timestamp.
debug!("future({})", lsn);
pitr_cutoff_lsn = gc_info.horizon_cutoff;
cutoff_horizon
}
LsnForTimestamp::Past(lsn) => {
debug!("past({})", lsn);
// conservative, safe default is to remove nothing, when we
// have no commit timestamp data available
*self.get_latest_gc_cutoff_lsn()
}
LsnForTimestamp::NoData(lsn) => {
debug!("nodata({})", lsn);
// conservative, safe default is to remove nothing, when we
// have no commit timestamp data available
*self.get_latest_gc_cutoff_lsn()
}
}
debug!("pitr_cutoff_lsn = {:?}", pitr_cutoff_lsn)
} else {
// If we don't have enough data to convert to LSN,
// play safe and don't remove any layers.
*self.get_latest_gc_cutoff_lsn()
}
} else {
// No time-based retention. (Some unit tests depend on garbage-collection
// working even when CLOG data is missing, so that find_lsn_for_timestamp()
// above doesn't work.)
pitr_cutoff_lsn = gc_info.horizon_cutoff;
}
gc_info.pitr_cutoff = pitr_cutoff_lsn;
// No time-based retention was configured. Set time-based cutoff to
// same as LSN based.
cutoff_horizon
};
// Grab the lock and update the values
*self.gc_info.write().unwrap() = GcInfo {
retain_lsns,
horizon_cutoff: cutoff_horizon,
pitr_cutoff,
};
Ok(())
}