[Virtio-fs] Large memory consumption by virtiofsd, suspecting fd's aren't being closed?

[Miklos Szeredi]

On Mon, Mar 22, 2021 at 5:47 PM Vivek Goyal <vgoyal at redhat.com> wrote:
>
> On Mon, Mar 22, 2021 at 05:09:32PM +0100, Miklos Szeredi wrote:
> > On Mon, Mar 22, 2021 at 6:52 AM Eric Ernst <eric_ernst at apple.com> wrote:
> > >
> > > Hey ya’ll,
> > >
> > > One challenge I’ve been looking at is how to setup an appropriate memory cgroup limit for workloads that are leveraging virtiofs (ie, running pods with Kata Containers). I noticed that memory usage of the daemon itself can grow considerably depending on the workload; though much more than I’d expect.
> > >
> > > I’m running workload that simply runs a build on kernel sources with -j3. In doing this, the source of the linux kernel are shared via virtiofs (no DAX), so as the build goes on, there are a lot of files opened, closed, as well as created. The rss memory of virtiofsd grows into several hundreds of MBs.
> > >
> > > When taking a look, I’m suspecting that virtiofsd is carrying out the opens, but never actually closing fds. In the guest, I’m seeing fd’s on the order of 10-40 for all the container processes as it runs, whereas I see the number of fds for virtiofsd continually increasing, reaching over 80,000 fds. I’m guessing this isn’t expected?
> >
> > The reason could be that guest is keeping a ref on the inodes
> > (dcache->dentry->inode) and current implementation of server keeps an
> > O_PATH fd open for each inode referenced by the client.
> >
> > One way to avoid this is to use the "cache=none" option, which forces
> > the client to drop dentries immediately from the cache if not in use.
> > This is not desirable if cache is actually in use.
> >
> > The memory use of the server should still be limited by the memory use
> > of the guest:  if there's memory pressure in the guest kernel, then it
> > will clean out caches, which results in the memory use decreasing in
> > the server as well.  If the server memory use looks unbounded, that
> > might be indicative of too much memory used for dcache in the guest
> > (cat /proc/slabinfo | grep ^dentry).     Can you verify?
>
> Hi Miklos,
>
> Apart from above, we identified one more issue on IRC. I asked Eric
> to drop caches manually in guest. (echo 3 > /proc/sys/vm/drop_caches)
> and while it reduced the fds open it did not seem to free up significant
> amount of memory.
>
> So question remains where is that memory. One possibility is that we
> have memory allocated for mapping arrays (inode and fd). These arrays
> only grow and never shrink. So they can lock down some memory.
>
> But still, lot of lo_inode memory should have been freed when
> echo 3 > /proc/sys/vm/drop_caches was done. Why all that did not
> show up in virtiofsd RSS usage, that's kind of little confusing.

Could be due to fragmentation.   I have no idea how the libc allocator works.

>
> cache=none is an alternative only if application is not using mmap.
> I think even kernel compilation now uses mmap towards the end and
> fails with cache=none.

Yes, I have plans to fix mmap with cache=none.

Thanks,
Miklos