On Tue, 2019-05-28 at 15:03 -0400, Chuck Lever wrote: > Following up on this. Now with even more data! > > > On May 6, 2019, at 4:41 PM, Chuck Lever <chuck.lever@xxxxxxxxxx> > > wrote: > > > > > > > On May 3, 2019, at 7:18 AM, Trond Myklebust <trondmy@xxxxxxxxx> > > > wrote: > > > > > > Allow more time for softirqd > > > > Have you thought about performance tests for this one? > > I tested this series on my 12-core two-socket client using a variety > of tests including iozone, fio, and fstests. The network under test > is 56Gb InfiniBand (TCP uses IPoIB). I tested both TCP and RDMA. > > With lock debugging and memory leak testing enabled, I did not see > any functional regressions or new leaks or crashes. Thus IMO this > series is "safe to apply." > > With TCP, I saw no change in performance between a "stock" kernel > and one with all five patches in this series applied, as, IIRC, > you predicted. > > The following discussion is based on testing with NFS/RDMA. > > With RDMA, I saw an improvement of 5-10% in IOPS rate between the > "stock" kernel and a kernel with the first four patches applied. When > the fifth patch is applied, I saw IOPS throughput significantly worse > than "stock" -- like 20% worse. > > I also studied average RPC execution time (the "execute" metric) with > the "stock" kernel, the one with four patches applied, and with the > one where all five are applied. The workload is 100% 4KB READs with > an iodepth of 1024 in order to saturate the transmit queue. > > With four patches, the execute time is about 2.5 msec faster (average > execution time is around 75 msec due to the large backlog this test > generates). With five patches, it's slower than "stock" by 12 msec. > > I also saw a 30 usec improvement in the average latency of > xprt_complete_rqst with the four patch series. > > As far as I can tell, the benefit of this series comes mostly from > the third patch, which changes spin_lock_bh(&xprt->transport_lock) to > spin_lock(&xprt->transport_lock). When the xprtiod work queue is > lowered in priority in 5/5, that benefit vanishes. > > I am still confused about why 5/5 is needed. I did not see any soft > lockups without this patch applied when using RDMA. Is the issue > with xprtsock's use of xprtiod for handling incoming TCP receives? > > I still have some things I'd like to look at. One thing I haven't > yet tried is looking at lock_stat, which would confirm or refute > my theory that this is all about the transport_lock, for instance. > OK. I can drop 5/5. The issue there was not about soft lockups. However since we were previously running most soft irqs as part of spin_unlock_bh(), the question was whether or not we would see more of them needing to move to softirqd. As far as I can see, your answer to that question is 'no' (at least for your system). Cheers Trond -- Trond Myklebust Linux NFS client maintainer, Hammerspace trond.myklebust@xxxxxxxxxxxxxxx
