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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Lieven (pl@kamp.de) wrote:
> 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
> > > > > memory
> > > > > 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
> > > > > release
> > > > >      pool. The choosen poolsize and the changed algorithm could lead to up
> > > > >      to
> > > > >      192 free coroutines with just a single iothread. Each of the
> > > > >      coroutines
> > > > >      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) cases.
> > 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 qemu-system-x86_64[555ab6f2c000+794000]
> 
> most of the time error 6. Sometimes error 7. segfault is near the sp.

A backtrace would be good.

Dave

> 
> 
> > 
> > > > > 2) Between Qemu 2.2.0 and 2.3.0 RCU was introduced which lead to delayed
> > > > > freeing
> > > > >      of memory. This lead to higher heap allocations which could not
> > > > >      effectively
> > > > >      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?
> 
> Peter
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK