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Re: virsh vol-download uses a lot of memory



On Wed, Jan 22, 2020 at 01:01:42PM +0100, Michal Privoznik wrote:
> On 1/22/20 11:11 AM, Michal Privoznik wrote:
> > On 1/22/20 10:03 AM, R. Diez wrote:
> > > Hi all:
> > > 
> > > I am using the libvirt version that comes with Ubuntu 18.04.3 LTS.
> > 
> > I'm sorry, I don't have Ubuntu installed anywhere to look the version
> > up. Can you run 'virsh version' to find it out for me please?
> 
> Nevermind, I've managed to reproduce with the latest libvirt anyway.
> 
> > 
> > > 
> > > I have written a script that backs up my virtual machines every
> > > night. I want to limit the amount of memory that this backup
> > > operation consumes, mainly to prevent page cache thrashing. I have
> > > described the Linux page cache thrashing issue in detail here:
> > > 
> > > http://rdiez.shoutwiki.com/wiki/Today%27s_Operating_Systems_are_still_incredibly_brittle#The_Linux_Filesystem_Cache_is_Braindead
> > > 
> > > 
> > > The VM virtual disk weighs 140 GB at the moment. I thought 500 MiB
> > > of RAM should be more than enough to back it up, so I added the
> > > following options to the systemd service file associated to the
> > > systemd timer I am using:
> > > 
> > >    MemoryLimit=500M
> > > 
> > > However, the OOM is killing "virsh vol-download":
> > > 
> > > Jan 21 23:40:00 GS-CEL-L kernel: [55535.913525] [  pid  ]   uid 
> > > tgid total_vm      rss pgtables_bytes swapents oom_score_adj name
> > > Jan 21 23:40:00 GS-CEL-L kernel: [55535.913527] [  13232]  1000
> > > 13232     5030      786    77824      103             0
> > > BackupWindows10
> > > Jan 21 23:40:00 GS-CEL-L kernel: [55535.913528] [  13267]  1000
> > > 13267     5063      567    73728      132             0
> > > BackupWindows10
> > > Jan 21 23:40:00 GS-CEL-L kernel: [55535.913529] [  13421]  1000
> > > 13421     5063      458    73728      132             0
> > > BackupWindows10
> > > Jan 21 23:40:00 GS-CEL-L kernel: [55535.913530] [  13428]  1000
> > > 13428 712847   124686  5586944   523997             0 virsh
> > > Jan 21 23:40:00 GS-CEL-L kernel: [55535.913532] oom-kill:constraint=CONSTRAINT_MEMCG,nodemask=(null),cpuset=/,mems_allowed=0,oom_memcg=/system.slice/VmBackup.service,task_memcg=/system.slice/VmBackup.service,task=virsh,pid=13428,uid=1000
> > > 
> > > Jan 21 23:40:00 GS-CEL-L kernel: [55535.913538] Memory cgroup out of
> > > memory: Killed process 13428 (virsh) total-vm:2851388kB,
> > > anon-rss:486180kB, file-rss:12564kB, shmem-rss:0kB
> > > 
> > > I wonder why "virsh vol-download" needs so much RAM. It does not get
> > > killed straight away, it takes a few minutes to get killed. It
> > > starts using a VMSIZE of around 295 MiB, which is not really frugal
> > > for a file download operation, but then it grows and grows.
> > 
> > This is very likely a memory leak somewhere.
> 
> Actually, it is not. It's caused by our design of the client event loop. If
> there are any incoming data, read as much as possible placing them at the
> end of linked list of incoming stream data (stream is a way that libvirt
> uses to transfer binary data). Problem is that instead of returning NULL to
> our malloc()-s once the limit is reached, kernel decides to kill us.
> 
> For anybody with libvirt insight: virNetClientIOHandleInput() ->
> virNetClientCallDispatch() -> virNetClientCallDispatchStream() ->
> virNetClientStreamQueuePacket().
> 
> 
> The obvious fix would be to stop processing incoming packets if stream has
> "too much" data cached (define "too much"). But this may lead to
> unresponsive client event loop - if the client doesn't pull data from
> incoming stream fast enough they won't be able to make any other RPC.

IMHO if they're not pulling stream data and still expecting to make
other RPC calls in a timely manner, then their code is broken.

Having said that, in retrospect I rather regret ever implementing our
stream APIs as we did. We really should have just exposed an API which
lets you spawn an NBD server associated with a storage volume, or
tunnelled NBD over libvirtd. The former is probably our best strategy
these days, now that NBD has native TLS support.

Regards,
Daniel
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