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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: Tue, 21 Jun 2016 17:12:57 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

Am 21.06.2016 um 15:18 schrieb Dr. David Alan Gilbert:
* Peter Lieven (address@hidden) wrote:

while upgrading from Qemu 2.2.0 to Qemu I noticed that the RSS memory 
usage has heavily increased.
We use hugepages so the RSS memory does not include VM memory. In Qemu 2.2.0 it 
used to be ~30MB per vServer
and increased to up to 300 - 400MB for Qemu (same with master). The 
memory increases over time, but seems
not to grow indefinetly. I tried to bisect, but had no result so far that made 
sense. I also tried valgrind / massif, but
valgrind does not see the allocation (at least at exit) and massif fails to 
rund due to - so it pretends - heap corruption.

Any help or ideas how to debug further would be appreciated.
I think I'd try stripping devices off; can you get a similar difference
to happen with a guest with no USB, no hugepages, no VGA and a simple
locally stored IDE disk?

Will do. VGA I already ruled out. Hugepages I can try, but its easier
to monitor the RSS size if the vServer memory is outside the RSS.

If you're having trouble bisecting is it possible it's a change
in one of the libraries it's linked against?

Same libraries. If I compile qemu-2.2.0 and qemu/master on the exactly
same machine I see the difference.

There was someone asking the other day on #qemu who had a setup that
was apparently using much more RAM than expected and we didn't
manage to track it down but I can't remember the version being used.

What I currently suspect are the following changes:
 - We changed the coroutine pool to a per thread model. I disabled the pool. 
This seems
   to cut the max used RSS to about 150MB which is still a lot more than 
 - I suspect that something (e.g. the object based device tree) is creating a 
lot of small allocations
   which create a massive overhead. I managed to get valgrind/massif running 
with attached debugger
   and took snapshots of the running VM. What I see is that kernel RSS size is 
much, much bigger than
   what massif sees as allocated memory. I talk of massif sees 5MB heap usage 
and the RSS size is 50MB or similar.
 - Changing the memory allocator to tcmalloc or jemalloc seems to relax the 
issue altough its not gone.
 - VGA memory seems to have been moved from VM memory into Heap. But thats a 
fixed 16MB allocation.

I will try to cut down devices as you proposed.


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