lwlock: Fix quadratic behavior with very long wait lists
Until now LWLockDequeueSelf() sequentially searched the list of waiters to see
if the current proc is still is on the list of waiters, or has already been
removed. In extreme workloads, where the wait lists are very long, this leads
to a quadratic behavior. #backends iterating over a list #backends
long. Additionally, the likelihood of needing to call LWLockDequeueSelf() in
the first place also increases with the increased length of the wait queue, as
it becomes more likely that a lock is released while waiting for the wait list
lock, which is held for longer during lock release.
Due to the exponential back-off in perform_spin_delay() this is surprisingly
hard to detect. We should make that easier, e.g. by adding a wait event around
the pg_usleep() - but that's a separate patch.
The fix is simple - track whether a proc is currently waiting in the wait list
or already removed but waiting to be woken up in PGPROC->lwWaiting.
In some workloads with a lot of clients contending for a small number of
lwlocks (e.g. WALWriteLock), the fix can substantially increase throughput.
This has been originally fixed for 16~ with a4adc31f6902 without a
backpatch, and we have heard complaints from users impacted by this
quadratic behavior in older versions as well.
Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>
Discussion: https://postgr.es/m/20221027165914.2hofzp4cvutj6gin@awork3.anarazel.de
Discussion: https://postgr.es/m/CALj2ACXktNbG=K8Xi7PSqbofTZozavhaxjatVc14iYaLu4Maag@mail.gmail.com
Backpatch-through: 12
Branch
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REL_12_STABLE
Details
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https://git.postgresql.org/pg/commitdiff/81038228582aa92718be2afcb7ddb67a5a19fa43
Author: Andres Freund <andres@anarazel.de>
Modified Files
--------------
src/backend/access/transam/twophase.c | 2 +-
src/backend/storage/lmgr/lwlock.c | 53 ++++++++++++++++++++---------------
src/backend/storage/lmgr/proc.c | 4 +--
src/include/storage/lwlock.h | 8 ++++++
src/include/storage/proc.h | 2 +-
5 files changed, 42 insertions(+), 27 deletions(-)