Re: [Qemu-devel] [PATCH v3] rcu: reduce more than 7MB heap memory by malloc_trim()

[Yang Zhong]

On Mon, Dec 04, 2017 at 08:26:29PM +0800, Shannon Zhao wrote:
> Hi Yang,
> 
> On 2017/12/4 20:03, Yang Zhong wrote:
> > On Fri, Dec 01, 2017 at 01:52:49PM +0100, Paolo Bonzini wrote:
> >> > On 01/12/2017 11:56, Yang Zhong wrote:
> >>> > >   This issue should be caused by much times of system call by 
> >>> > > malloc_trim(),
> >>> > >   Shannon's test script include 60 scsi disks and 31 ioh3420 devices. 
> >>> > > We need 
> >>> > >   trade-off between VM perforamance and memory optimization. Whether 
> >>> > > below 
> >>> > >   method is suitable?
> >>> > > 
> >>> > >   int num=1;
> >>> > >   ......
> >>> > > 
> >>> > >   #if defined(CONFIG_MALLOC_TRIM)
> >>> > >         if(!(num++%5))
> >>> > >         {
> >>> > >              malloc_trim(4 * 1024 * 1024);
> >>> > >         }
> >>> > >   #endif
> >>> > >  
> >>> > >   Any comments are welcome! Thanks a lot!
> >> > 
> >> > Indeed something like this will do, perhaps only trim once per second?
> >> > 
> >   Hello Paolo,
> > 
> >   Thanks for comments!
> >   If we do trim once per second, maybe the frequency is a little high, 
> > what'e
> >   more, we need maintain one timer to call this, this also cost cpu 
> > resource.
> > 
> >   I added the log and did the test here with my test qemu command, when VM 
> > bootup,
> >   which did more than 600 times free operations and 9 times memory trim in 
> > rcu 
> >   thread. If i use our ClearContainer qemu command, the memory trim will 
> > down 
> >   to 6 times. As for Shannon's test command, the malloc trim number will 
> > abosultly 
> >   increse.
> > 
> >   In my above method, the trim is only executed in the multiple of 5, which 
> > will
> >   reduce trim times and do not heavily impact VM bootup performance. 
> > 
> >   I also want to use synchronize_rcu() and free() to replace call_rcu(), 
> > but this
> >   method serialize to malloc() and free(), which will reduce VM performance.
> > 
> >   The ultimate aim is to reduce trim system call during the VM bootup and 
> > running.
> >   It's appreciated that if you have better suggestions.
> 
> Maybe we can provide a QMP command or something else for user to trim
> the heap manually like the kernel sysfs interface
> /proc/sys/vm/drop_caches which provides an interface for user to drop
> the caches.
> So let user to decide whether it needs to trim the heap.
>
  Hello Shannon,

  Thanks for your comments!
  This is also a good solution by QMP interface, but this is only suitable for 
few VMs.
  If there are millions of VMs in CSP(clouds of provider), it is very hard to 
operate.
  Thanks!

  Regards,

  Yang     

 
> Thanks,
> -- 
> Shannon