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.
Anybody got any ideas?
Michal
