[Xen-devel] [for-4.8 PATCH] xen: credit2: make runqueues be per-socket by default.

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

Hi,

This patch changes the way in which runqueue are arranged by default in
Credit2. Despite being so late, I think this should go in Xen 4.8, and here's
why.

So, first of all, runqueues in Credit2 can be arranged in various ways, just by
means of a Xen boot parameter. Right now, the default is one runqueue per-core.
I think we should switch that to one runqueue per-socket.

In fact, all the testing and the benchmarking that I have done before sending
all the various patch series about Credit2 within this release cycle, happened
with per-socket runqueues.

The numbers reported here, for instance, show how per-socket is better:

https://lists.xen.org/archives/html/xen-devel/2016-06/msg02287.html

Pretty much all the other results we have tell the same.

Most of Anshul's benchmarks, the ones he presented at XenSummit, the ones he
has run after that, and the ones he is running now, use per-socket runqueues.

http://www.slideshare.net/xen_com_mgr/xpds16-scope-and-performance-of-credit2-scheduler-anshul-makkar-ctirix-systems-uk-ltd

The reason why we did go for per-core in 4.7, was to introduce some form of
hyperthreading support. Now, we have hyperthreading support, independently from
how runqueues are organized (9bb9c7388 "xen: credit2: implement true SMT
support").

So, in summary, the per-socket ruqneueue configuration is _BOTH_ more tested,
and provide better performance.

The reason why I'm proposing this only now is basically because I thought I had
sent a patch for that already, but I've recently discovered that I must have
took such patch off from one of the series at the last minute. And since then,
I've always forgot to double check... :-(

As it is easy to check, the change is entirely self contained within Credit2,
and bears no risks.

Finally, note that, given the timing (and the fact that he is away), I spoke
with George out of band, and he told me I can put his Ack on the patch. Of
course, we defer to Wei to decide if this is still acceptable.

Thanks, and sorry, and Regards, :-)
Dario
---
Dario Faggioli (1):
      xen: credit2: make runqueues be per-socket by default.

 docs/misc/xen-command-line.markdown |    2 +-
 xen/common/sched_credit2.c          |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel