Re: [Cluster-devel] dlm: Remove/bypass astd

[Christine Caulfield]

On 17/02/10 20:29, David Teigland wrote:
> On Wed, Feb 17, 2010 at 01:23:39PM +0000, Steven Whitehouse wrote:
> > While investigating Red Hat bug #537010 I started looking at the dlm's astd
> > thread. The way in which the "cast" and "bast" requests are queued looked
> > as if it might cause reordering since the "bast" requests are always
> > delivered after any pending "cast" requests which is not always the
> > correct ordering. This patch doesn't fix that bug, but it will prevent any
> > races in that bit of code, and the performance benefits are also well
> > worth having.
> >
> > I noticed that astd seems to be extraneous to requirements. The notifications
> > to astd are already running in process context, so they could be delivered
> > directly. That should improve smp performance since all the notifications
> > would no longer be funneled through a single thread.
> >
> > Also, the only other function of astd seemed to be stopping the delivery
> > of these notifications during recovery. Since, however, the notifications
> > which are intercepted at recovery time are neither modified, nor filtered
> > in any way, the only effect is to delay notifications for no obvious reason.
> >
> > I thought that probably removing the astd thread and delivering the "cast"
> > and "bast" notifications directly would improve performance due to the
> > elimination of a scheduling delay. I wrote a small test module which
> > creates a dlm lock space, and does 100,000 NL ->  EX ->  NL lock conversions.
> >
> > Having run this test 10 times each on a 2.6.33-rc8 kernel and then the modified
> > kernel including this patch, I got the following results:
> >
> > Original: Avg time 24.62 us per conversion (NL ->  EX ->  NL)
> > Modified: Avg time 9.93 us per conversion
> >
> > Which is a fairly dramatic speed up. Please consider applying this patch.
> > I've tested it in both clustered and single node GFS2 configurations. The test
> > figures are from a single node configuration which was a deliberate choice
> > in order to avoid any effects of network latency.
>
> Wow, there's no chance I'm going to even consider something like this.
> This would be a huge change in how the dlm has always operated, and would
> surely introduce very serious and hard to identify bugs (and ones that may
> not appear for a long time afterward).  Given that there's *no problem* with
> the current method that has worked well for years, any change would be
> completely crazy.

I think total dismissal of the idea is unreasonable. At least give it a 
chance, and test it for a while. Your argument is valid for not 
including it in a stable release as yet, but not for ignoring it 
completely.

Here we have a chance to remove some code, a kernel thread AND improve 
performance hugely. Just saying "it's always worked like that and we're 
not changing it" is the crazy argument IMHO.

Chrissie
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