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

[Shannon Zhao]

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.

Thanks,
-- 
Shannon