Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default

[Gonglei (Arei)]

> -----Original Message-----
> From: address@hidden [mailto:address@hidden
> Sent: Wednesday, November 29, 2017 1:56 PM
> To: Gonglei (Arei)
> Cc: Eduardo Habkost; Denis V. Lunev; longpeng; Michael S. Tsirkin; Denis
> Plotnikov; address@hidden; address@hidden; address@hidden;
> huangpeng; Zhaoshenglong
> Subject: Re: [Qemu-devel] [PATCH] i386: turn off l3-cache property by default
> 
> On Wed, Nov 29, 2017 at 01:57:14AM +0000, Gonglei (Arei) wrote:
> > > On Tue, Nov 28, 2017 at 11:20:27PM +0300, Denis V. Lunev wrote:
> > > > On 11/28/2017 10:58 PM, Eduardo Habkost wrote:
> > > > > On Fri, Nov 24, 2017 at 04:26:50PM +0300, Denis Plotnikov wrote:
> > > > >> Commit 14c985cffa "target-i386: present virtual L3 cache info for
> vcpus"
> > > > >> introduced and set by default exposing l3 to the guest.
> > > > >>
> > > > >> The motivation behind it was that in the Linux scheduler, when waking
> up
> > > > >> a task on a sibling CPU, the task was put onto the target CPU's
> runqueue
> > > > >> directly, without sending a reschedule IPI.  Reduction in the IPI 
> > > > >> count
> > > > >> led to performance gain.
> > > > >>
> >
> > Yes, that's one thing.
> >
> > The other reason for enabling L3 cache is the performance of accessing
> memory.
> 
> I guess you're talking about the super-smart buffer size tuning glibc
> does in its memcpy and friends.  We try to control that with an atomic
> test for memcpy, and we didn't notice a difference.  We'll need to
> double-check...
> 
> > We tested it by Stream benchmark, the performance is better with
> L3-cache=on.
> 
> This one: https://www.cs.virginia.edu/stream/ ?  Thanks, we'll have a
> look, too.
> 
Yes. :)

Thanks,
-Gonglei