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

[Daniel P. Berrange]

On Mon, Dec 04, 2017 at 08:03:22PM +0800, 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.

Does configuring QEMU to use tcmalloc or jemalloc instead of glibc's malloc
give you the performance & menmory usage that you require? If so, it might
not be worth bothering to hack around problems in glibc's malloc at all.


Regards,
Daniel
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