Re: [PATCH] mm: page_alloc: High-order per-cpu page allocator v7

From: Jesper Dangaard Brouer
Date: Thu Dec 08 2016 - 03:22:43 EST

On Wed, 7 Dec 2016 23:25:31 +0000
Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote:

> On Wed, Dec 07, 2016 at 09:19:58PM +0000, Mel Gorman wrote:
> > At small packet sizes on localhost, I see relatively low page allocator
> > activity except during the socket setup and other unrelated activity
> > (khugepaged, irqbalance, some btrfs stuff) which is curious as it's
> > less clear why the performance was improved in that case. I considered
> > the possibility that it was cache hotness of pages but that's not a
> > good fit. If it was true then the first test would be slow and the rest
> > relatively fast and I'm not seeing that. The other side-effect is that
> > all the high-order pages that are allocated at the start are physically
> > close together but that shouldn't have that big an impact. So for now,
> > the gain is unexplained even though it happens consistently.
> >
> Further investigation led me to conclude that the netperf automation on
> my side had some methodology errors that could account for an artifically
> low score in some cases. The netperf automation is years old and would
> have been developed against a much older and smaller machine which may be
> why I missed it until I went back looking at exactly what the automation
> was doing. Minimally in a server/client test on remote maching there was
> potentially higher packet loss than is acceptable. This would account why
> some machines "benefitted" while others did not -- there would be boot to
> boot variations that some machines happened to be "lucky". I believe I've
> corrected the errors, discarded all the old data and scheduled a rest to
> see what falls out.

I guess you are talking about setting the netperf socket queue low
(+256 bytes above msg size), that I pointed out in[1]. I can see from
GitHub-mmtests-commit[2] "netperf: Set remote and local socket max
buffer sizes", that you have removed that, good! :-)

>From the same commit[2] I can see you explicitly set (local+remote):

sysctl net.core.rmem_max=16777216
sysctl net.core.wmem_max=16777216

Eric do you have any advice on this setting?

And later[4] you further increase this to 32MiB. Notice that the
netperf UDP_STREAM test will still use the default value from:
net.core.rmem_default = 212992.

(To Eric) Mel's small UDP queues also interacted badly with Eric and
Paolo's UDP improvements, which was fixed in net-next commit[3]
363dc73acacb ("udp: be less conservative with sock rmem accounting").

Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat