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Re: [Qemu-devel] [PATCH 00/15] optimize Qemu RSS usage

From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH 00/15] optimize Qemu RSS usage
Date: Tue, 28 Jun 2016 14:33:02 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

Am 28.06.2016 um 14:29 schrieb Paolo Bonzini:
Am 28.06.2016 um 13:37 schrieb Paolo Bonzini:
On 28/06/2016 11:01, Peter Lieven wrote:
I recently found that Qemu is using several hundred megabytes of RSS
more than older versions such as Qemu 2.2.0. So I started tracing
memory allocation and found 2 major reasons for this.

1) We changed the qemu coroutine pool to have a per thread and a global
     pool. The choosen poolsize and the changed algorithm could lead to up
     192 free coroutines with just a single iothread. Each of the
     in the pool each having 1MB of stack memory.
But the fix, as you correctly note, is to reduce the stack size.  It
would be nice to compile block-obj-y with -Wstack-usage=2048 too.
To reveal if there are any big stack allocations in the block layer?
Yes.  Most should be fixed by now, but a handful are probably still there.
(definitely one in vvfat.c).

As it seems reducing to 64kB breaks live migration in some (non reproducible) 
Does it hit the guard page?

How would that look like? I get segfaults like this:

segfault at 7f91aa642b78 ip 0000555ab714ef7d sp 00007f91aa642b50 error 6 in 

most of the time error 6. Sometimes error 7. segfault is near the sp.

2) Between Qemu 2.2.0 and 2.3.0 RCU was introduced which lead to delayed
     of memory. This lead to higher heap allocations which could not
     be returned to kernel (most likely due to fragmentation).
I agree that some of the exec.c allocations need some care, but I would
prefer to use a custom free list or lazy allocation instead of mmap.
This would only help if the elements from the free list would be allocated
using mmap? The issue is that RCU delays the freeing so that the number of
concurrent allocations is high and then a bunch is freed at once. If the memory
was malloced it would still have caused trouble.
The free list should improve reuse and fragmentation.  I'll take a look at
lazy allocation of subpages, too.

Ok, that would be good. And for the PhsyPageMap we use mmap and try to avoid
the realloc?


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