On Tue, Nov 28 2017, Mike Marion wrote: > On Tue, Nov 28, 2017 at 07:43:05AM +0800, Ian Kent wrote: > >> I think the situation is going to get worse before it gets better. >> >> On recent Fedora and kernel, with a large map and heavy mount activity >> I see: >> >> systemd, udisksd, gvfs-udisks2-volume-monitor, gvfsd-trash, >> gnome-settings-daemon, packagekitd and gnome-shell >> >> all go crazy consuming large amounts of CPU. > > Yep. I'm not even worried about the CPU usage as much (yet, I'm sure > it'll be more of a problem as time goes on). We have pretty huge > direct maps and our initial startup tests on a new host with the link vs > file took >6 hours. That's not a typo. We worked with Suse engineering > to come up with a fix, which should've been pushed here some time ago. > > Then, there's shutdowns (and reboots). They also took a long time (on > the order of 20+min) because it would walk the entire /proc/mounts > "unmounting" things. Also fixed now. That one had something to do in > SMP code as if you used a single CPU/core, it didn't take long at all. > > Just got a fix for the suse grub2-mkconfig script to fix their parsing > looking for the root dev to skip over fstype autofs > (probe_nfsroot_device function). > >> The symlink change was probably the start, now a number of applications >> now got directly to the proc file system for this information. >> >> For large mount tables and many processes accessing the mount table >> (probably reading the whole thing, either periodically or on change >> notification) the current system does not scale well at all. > > We use Clearcase in some instances as well, and that's yet another thing > adding mounts, and its startup is very slow, due to the size of > /proc/mounts. > > It's definitely something that's more than just autofs and probably > going to get worse, as you say. If we assume that applications are going to want to read /proc/self/mount* a log, we probably need to make it faster. I performed a simple experiment where I mounted 1000 tmpfs filesystems, copied /proc/self/mountinfo to /tmp/mountinfo, then ran 4 for loops in parallel catting one of these files to /dev/null 1000 times. On a single CPU VM: For /tmp/mountinfo, each group of 1000 cats took about 3 seconds. For /proc/self/mountinfo, each group of 1000 cats took about 14 seconds. On a 4 CPU VM /tmp/mountinfo: 1.5secs /proc/self/mountinfo: 3.5 secs Using "perf record" it appears that most of the cost is repeated calls to prepend_path, with a small contribution from the fact that each read only returns 4K rather than the 128K that cat asks for. If we could hang a cache off struct mnt_namespace and use it instead of iterating the mount table - using rcu and ns->event to ensure currency - we should be able to minimize the cost of this increased use of /proc/self/mount*. I suspect that the best approach would be implement a cache at the seq_file level. One possible problem might be if applications assume that a read will always return a whole number of lines (it currently does). To be sure we remain safe, we would only be able to use the cache for a read() syscall which reads the whole file. How big do people see /proc/self/mount* getting? What size reads does 'strace' show the various programs using to read it? Thanks, NeilBrown
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