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Re: virsh vol-download uses a lot of memory



On 1/22/20 1:18 PM, Daniel P. Berrangé wrote:
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.

This is virsh that we are talking about. It's not some random application.

And I am able to limit virsh mem usage to "just" 100MiB with one well placed usleep() - to slow down putting incoming stram packets onto the queue:

diff --git i/src/rpc/virnetclientstream.c w/src/rpc/virnetclientstream.c
index f904eaba31..cfb3f225f2 100644
--- i/src/rpc/virnetclientstream.c
+++ w/src/rpc/virnetclientstream.c
@@ -358,6 +358,7 @@ int virNetClientStreamQueuePacket(virNetClientStreamPtr st,
     virNetClientStreamEventTimerUpdate(st);

     virObjectUnlock(st);
+    usleep(1000);
     return 0;
 }


But any attempt I've made to ignore POLLIN if stream queue is longer than say 8 packets was unsuccessful (the code still read incoming packets and placed them into the queue). I blame passing the bucket algorithm for that (rather than my poor skills :-P).


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.

Yeah, but IIRC NBD wasn't a thing back then, was it?

Michal


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