Hi All,
I've been trying to debug a problem with PVFS, where performance degrades slowly with a long-lived (weeks and months) PVFS volume. The degradation is significant -- simple metadata operations are an order of magnitude slower after a month or so. The behavior turns out to only occur with the VFS and pvfs2-client daemon: performance of the admin tools (pvfs2-touch, pvfs2-rm, etc.) to the same set of servers remains good. Restarting the client daemon also fixes the problem, suggesting that the long-lived open sockets are somehow the cause. The slowness also appears to be at the servers not the clients: the same kernel module and client daemon to a different filesystem and set of servers doesn't exhibit the performance degradation.
Also, I should mention that the system config is a little different than usual. We have IO nodes mounting and unmounting the PVFS volume (and stopping the client daemon) with each user's job, which is fairly frequent, while on the login nodes, the volume remains mounted for a long time (and where the performance degrades).
Our hunch here is that epoll or our use of epoll on the servers is somehow to blame. Maybe the file descriptors opened on the server for pvfs2-client-core are getting pushed down further and further into the epoll set, which for some reason is growing with new connections coming and going. This might be the case if we were failing to remove sockets from the set on disconnect, for example. It doesn't look like that's happening though, at least for normal disconnects.
Its a PITA to debug, because the servers have to remain running for a long time (and the clients have to remain mounted) for the problem to be visible. Rob suggested I use strace on the servers to see what epoll was doing, and that showed some interesting results. Basically, it looks like epoll_wait takes significantly longer when clients are doing operations over the VFS, rather than with the pvfs2 admin tools. Also, strace reported epoll_ctl(..., EPOLL_CTL_ADD, ...)) getting called a few times, even for the VFS ops, and in those cases its returning EEXISTS.
I noticed that we add a socket to the epoll set whenever we get a new connection, or a read or write is posted (enqueue_operation), but we only remove the socket from the epoll set on errors or disconnects. So why are we adding it for reads and writes? Any connected socket should already be in the set, no? I think this may be why I'm seeing EEXISTS with strace.
Also, is it safe to check the error from epoll_ctl in BMI_socket_collection_[add|remove]?
And finally, assuming PVFS is actually using epoll calls properly, does anyone know of epoll bugs on a SUSE 2.6.5 kernel that would cause epoll_ctl(..., EPOLL_CTL_DEL, ....) to not do what its meant to? Googling epoll and SUSE 2.6.5 isn't turning up anything...
Thanks, -sam _______________________________________________ Pvfs2-developers mailing list [email protected] http://www.beowulf-underground.org/mailman/listinfo/pvfs2-developers
