> On Nov 19, 2024, at 10:09 AM, Trond Myklebust <trondmy@xxxxxxxxxxxxxxx> wrote: > > On Tue, 2024-11-19 at 06:45 -0500, Jeff Layton wrote: >> We attempted to implement the "delstid" draft for v6.13, but have had >> to drop the patches for it. After merge, we got a couple of reports >> of >> a performance issue due to the OPEN_XOR_DELEGATION patch: >> >> >> https://lore.kernel.org/linux-nfs/202409161645.d44bced5-oliver.sang@xxxxxxxxx/ >> >> Once we enable OPEN_XOR_DELEGATION support, the fsmark "App Overhead" >> statistic spikes significantly. The kernel patch for this is very >> simple, and doesn't seem likely to cause a performance issue on its >> own. My theory is that this test is one that causes the client to >> return the delegation, and since it doesn't have an open stateid, it >> has to reestablish one during the test run, and that causes the app >> overhead stat to spike. >> >> Trond, Tom, Mike -- I know that the HS Anvil has support for >> OPEN_XOR_DELEGATION. If you run the fsmark test against it with that >> support both enabled and disabled (either on the client or server >> side), do you see a similar spike in "App Overhead"? >> >> If so, then I suspect we need to consider limiting the use of that >> flag >> in some cases. I have no idea what heuristic we'd use to decide this >> though. > > As already stated when we discussed this at Bakeathon: the server is > still in charge of heuristics w.r.t. whether or not there may be > contention for the file. The OPEN_XOR_DELEGATION flag changes nothing > in that respect. fsmark is a single-client test. There should be no contention for any files during this test. > Yes, I'm sure you can find tests which cause recalls of delegations, > and those will be marginally slower when the client has to re-establish > an open stateid. The fsmark result regressed 92%. > However the issue with those tests is that they are > deliberately setting up a situation where the server ideally shouldn't > be handing out a delegation at all. > > Furthermore, this is no different than a situation where the client > used a delegation to cache the open (i.e. avoid sending an OPEN call) > after the application closed the file and then later re-opened it. > So the point is that this is not a situation that is unique to > OPEN_XOR_DELEGATION. It is just a consequence of the client's ability > to cache open state. The regression was bisected to Jeff's XOR patch on two separate occasions. This does indeed appear to be a situation that is unique to OPEN_XOR_DELEGATION. It's possible that our theory of the failure is wrong. As developers of the only other server implementation of OPEN_XOR_DELEGATION, can Hammerspace help us troubleshoot this issue? -- Chuck Lever
