Subject: Re: Proposal of tunable fix for scalability of 8.4
From: Simon Riggs <simon@2ndQuadrant.com>
To: Matthew Wakeling <matthew@flymine.org>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
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Date: Wed, 18 Mar 2009 12:06:49 +0000
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On Wed, 2009-03-18 at 11:45 +0000, Matthew Wakeling wrote:
> On Wed, 18 Mar 2009, Simon Riggs wrote:
> > I agree with that, apart from the "granting no more" bit.
> >
> > The most useful behaviour is just to have two modes:
> > * exclusive-lock held - all other x locks welcome, s locks queue
> > * shared-lock held - all other s locks welcome, x locks queue
> 
> The problem with making all other locks welcome is that there is a 
> possibility of starvation. Imagine a case where there is a constant stream 
> of shared locks - the exclusive locks may never actually get hold of the 
> lock under the "all other shared locks welcome" strategy. 

That's exactly what happens now. 

> Likewise with the reverse.

I think it depends upon how frequently requests arrive. Commits cause X
locks and we don't commit that often, so its very unlikely that we'd see
a constant stream of X locks and prevent shared lockers.


Some comments from an earlier post on this topic (about 20 months ago):

Since shared locks are currently queued behind exclusive requests
when they cannot be immediately satisfied, it might be worth
reconsidering the way LWLockRelease works also. When we wake up the
queue we only wake the Shared requests that are adjacent to the head of
the queue. Instead we could wake *all* waiting Shared requestors.

e.g. with a lock queue like this:
(HEAD)  S<-S<-X<-S<-X<-S<-X<-S
Currently we would wake the 1st and 2nd waiters only. 

If we were to wake the 3rd, 5th and 7th waiters also, then the queue
would reduce in length very quickly, if we assume generally uniform
service times. (If the head of the queue is X, then we wake only that
one process and I'm not proposing we change that). That would mean queue
jumping right? Well thats what already happens in other circumstances,
so there cannot be anything intrinsically wrong with allowing it, the
only question is: would it help? 

We need not wake the whole queue, there may be some generally more
beneficial heuristic. The reason for considering this is not to speed up
Shared requests but to reduce the queue length and thus the waiting time
for the Xclusive requestors. Each time a Shared request is dequeued, we
effectively re-enable queue jumping, so a Shared request arriving during
that point will actually jump ahead of Shared requests that were unlucky
enough to arrive while an Exclusive lock was held. Worse than that, the
new incoming Shared requests exacerbate the starvation, so the more
non-adjacent groups of Shared lock requests there are in the queue, the
worse the starvation of the exclusive requestors becomes. We are
effectively randomly starving some shared locks as well as exclusive
locks in the current scheme, based upon the state of the lock when they
make their request. The situation is worst when the lock is heavily
contended and the workload has a 50/50 mix of shared/exclusive requests,
e.g. serializable transactions or transactions with lots of
subtransactions.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support