On Mon, Mar 22, 2021 at 12:47:04PM -0400, Vivek Goyal 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 <[email protected]> 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.
Are you including "RssShmem" in "RSS usage"? If so, that could be misleading. When virtiofsd[-rs] touches pages that reside in the memory mapping that's shared with QEMU, those pages are accounted in the virtiofsd[-rs] process's RssShmem too. In other words, the RSS value of the virtiofsd[-rs] process may be overinflated because it includes pages that are actually shared (there's no a second copy of them) with the QEMU process. This can be observed using a tool like "smem". Here's an example - This virtiofsd-rs process appears to have a RSS of ~633 MiB root 13879 46.1 7.9 8467492 649132 pts/1 Sl+ 11:33 0:52 ./target/debug/virtiofsd-rs root 13947 69.3 13.4 5638580 1093876 pts/0 Sl+ 11:33 1:14 qemu-system-x86_64 - In /proc/13879/status we can observe most of that memory is actually RssShmem: RssAnon: 9624 kB RssFile: 5136 kB RssShmem: 634372 kB - In "smem", we can see a similar amount of RSS, but the PSS is roughly half the size because "smem" is splitting it up between virtiofsd-rs and QEMU: [root@localhost ~]# smem -P virtiofsd-rs -P qemu PID User Command Swap USS PSS RSS 13879 root ./target/debug/virtiofsd-rs 0 13412 337019 662392 13947 root qemu-system-x86_64 -enable- 0 434224 760096 1094392 - If we terminate the virtiofsd-rs process, the output of "smem" now shows that QEMU's PSS has grown to account for the PSS that was previously assigned to virtiofsd-rs too, so we can confirm that was memory shared between both processes. PID User Command Swap USS PSS RSS 13947 root qemu-system-x86_64 -enable- 0 1082656 1084966 1095692 Just to be 100% sure, I've also run "heaptrack" on a virtiofsd-rs instance, and can confirm that the actual heap usage of the process was around 5-6 MiB. Sergio.
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