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Re: [Qemu-devel] Qemu and heavily increased RSS usage

From: Peter Lieven
Subject: Re: [Qemu-devel] Qemu and heavily increased RSS usage
Date: Fri, 24 Jun 2016 11:53:41 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

Am 24.06.2016 um 11:37 schrieb Stefan Hajnoczi:
> On Wed, Jun 22, 2016 at 09:56:06PM +0100, Peter Maydell wrote:
>> On 22 June 2016 at 20:55, Peter Lieven <address@hidden> wrote:
>>> What makes the coroutine pool memory intensive is the stack size of 1MB per
>>> coroutine. Is it really necessary to have such a big stack?
>> That reminds me that I was wondering if we should allocate
>> our coroutine stacks with MAP_GROWSDOWN (though if we're
>> not actually using 1MB of stack then it's only going to
>> be eating virtual memory, not necessarily real memory.)
> Yes, MAP_GROWSDOWN will not reduce RSS.

Yes, I can confirm just tested...

> It's possible that we can reduce RSS usage of the coroutine pool but it
> will require someone to profile the pool usage patterns.

It would be interesting to see what stack size we really need. Is it possible
to automatically detect this value (at compile time?)

I can also confirm that the coroutine pool is the second major RSS user beside
heap fragmentation.

Lowering the mmap threshold of malloc to about 32k also gives good results.
In this case there are very few active mappings in the running vServer, but the
RSS is still at about 50MB (without coroutine pool). Maybe it would be good
to identify which parts of Qemu malloc lets say >16kB and convert them to mmap
if it is feasible.


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