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_inc... >>> >>> >>> 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.