-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 12/14/2011 03:32 PM, Dave Chinner wrote: > On Wed, Dec 14, 2011 at 02:24:32PM -0500, Jeff Mahoney wrote: >> -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 >> >> On 12/14/2011 01:27 PM, Mark Fasheh wrote: >>> Hey Christoph, >>> >>> On Tue, Dec 13, 2011 at 09:40:40PM -0500, Christoph Hellwig >>> wrote: >>>> Can you explain the story behid this patch in SLES11SP1? >>> >>> We were looking at some performance issues and needed a bit >>> more information on the amount of time spent in ilock. I can >>> give you more specifics if you want, I just have to dig up the >>> e-mails (it's been a while). >> >> That's pretty much the explanation. With heavy reader load, >> buffered writes were stalling for 80 ms and sometimes longer. I >> suspected it was contention on the ilock and the tracing with >> that patch demonstrated a delay there. Since we were chasing a >> similar issue at another site, it seemed worthwhile to just keep >> it around. We're still tracking down the cause. I'm not sure if >> more recent kernels have the same issue as there's been quite a >> lot of churn. > > I'm not surprised - there's nothing really guaranteeing bound > shared vs exclusive access to the ilock. It's all down to the > read/write bias of the rwsem - readers will hold off the writer for > some time. Still, it would be nice to see a trace from such a > holdoff to confirm this is actually the case... Sure, let me dig it up. We do actually have real results from this workload with the trace running. > FWIW, if you have an app that requires concurrent, low latency > reads and writes to the same file, that's what the XFS Direct IO > was designed for - in most cases the iolock is taken in shared mode > for both read and write, and so such hold-offs don't generally > happen... You don't need to convince me. I agree with you. :) - -Jeff - -- Jeff Mahoney SUSE Labs -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.18 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQIcBAEBAgAGBQJO6SZeAAoJEB57S2MheeWyzfgP/3gbX++mlm3pY8iRlWkqEcy4 77OBJyMn0jxUqQkLw3nmrCcIMVJxKREJhtBoNl93mBdN8HZvx1X7nTUst769Ksf/ NsjQd3NMyv/pymjuJBB81APnyc2diM/fz/t5f1gh4xHljKG9rUJS8ogRQ65QBbMG kDbwtgaXYMZpUpf/GAF7Q0jxefNZbs82BfSsGqDH6e+fdymqD9/GWewyIFCl3tVR yOk80v4izDwlgtwtm/anq18nqePZl8M3ktwtucpAR/cxZ79nOryGxuqaD5r7GOam bo/29A5x0UKptsxHKnEvf6JIlznVWR6RzheyV/A6d3gPFkMa8550NBFMmWObjh7h DA/fcsZFhzD1LZI9BXU58ib6nT3kmtQMHmdsuwPsX5XPLzdymMNFD/yUDP7vhMWC OR1kkLPBSGeCFK7m0IeScO+jCnVsadhzkeS3rMjbUYCSw3+4OGf21sAJnfsMasPb Z2ZbbSZdUGItb8bK1lx7WEwST8KC5UYe2n3LIoH8CJ8qH+FrzX0pQKx+Gm2SVoMK nZZKDbtwN9+ZWRs2p3tEuT34sb/XGFV1XnEBPAv272XAjZKvjwBD8JZF92fQxl8a e2yjy7+EsqmZXNguTcs+KhLJ69QkKkg6J0rozPgUNHmA+KGhSDCgRI2kMCPDL3MO uKZ2umow6YYTofS6mBp8 =zmC1 -----END PGP SIGNATURE----- _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
