Heikki Linnakangas bfa1d91612 Introduce RCU, and use it to protect latest_gc_cutoff_lsn. ...

`latest_gc_cutoff_lsn` tracks the cutoff point where GC has been
performed. Anything older than the cutoff might already have been GC'd
away, and cannot be queried by get_page_at_lsn requests. It's
protected by an RWLock. Whenever a get_page_at_lsn requests comes in,
it first grabs the lock and reads the current `latest_gc_cutoff`, and
holds the lock it until the request has been served. The lock ensures
that GC doesn't start concurrently and remove page versions that we
still need to satisfy the request.

With the lock, get_page_at_lsn request could potentially be blocked
for a long time.  GC only holds the lock in exclusive mode for a short
duration, but depending on how whether the RWLock is "fair", a read
request might be queued behind the GC's exclusive request, which in
turn might be queued behind a long-running read operation, like a
basebackup. If the lock implementation is not fair, i.e. if a reader
can always jump the queue if the lock is already held in read mode,
then another problem arises: GC might be starved if a constant stream
of GetPage requests comes in.

To avoid the long wait or starvation, introduce a Read-Copy-Update
mechanism to replace the lock on `latest_gc_cutoff_lsn`. With the RCU,
reader can always read the latest value without blocking (except for a
very short duration if the lock protecting the RCU is contended;
that's comparable to a spinlock). And a writer can always write a new
value without waiting for readers to finish using the old value. The
old readers will continue to see the old value through their guard
object, while new readers will see the new value.

This is purely theoretical ATM, we don't have any reports of either
starvation or blocking behind GC happening in practice. But it's
simple to fix, so let's nip that problem in the bud.

2022-08-29 11:23:37 +03:00

..

src

Introduce RCU, and use it to protect latest_gc_cutoff_lsn.

2022-08-29 11:23:37 +03:00

Cargo.toml

Remove unused dependency

2022-08-27 18:14:33 +03:00