Messages in this thread | | | Date | Fri, 25 Apr 2008 10:38:11 +0200 | From | Jens Axboe <> | Subject | Re: [RFC][PATCH 0/3] Skip I/O merges when disabled |
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On Thu, Apr 24 2008, Alan D. Brunelle wrote: > Jens Axboe wrote: > > On Wed, Apr 23 2008, Alan D. Brunelle wrote: > >> The block I/O + elevator + I/O scheduler code spends a lot of time > >> trying to merge I/Os -- rightfully so under "normal" circumstances. > >> However, if one were to know that the incoming I/O stream was /very/ > >> random in nature, the cycles are wasted. (This can be the case, for > >> example, during OLTP-type runs.) > >> > >> This patch stream adds a per-request_queue tunable that (when set) > >> disables merge attempts, thus freeing up a non-trivial amount of CPU cycles. > >> > >> I'll be doing some more benchmarking, but this is a representative set > >> of data on a two-way Opteron box w/ 4 SATA drives. 'fio' was used to > >> generate random 4k asynchronous direct I/Os over the 128GiB of each SATA > >> drive. Oprofile was used to collect the results, and we collected > >> CPU_CLK_UNHALTED (CPU) and DATA_CACHE_MISSES (DCM) events. The data > >> extracted below shows both the percentage for all samples (including > >> non-kernel) as well as just those from the block I/O layer + elevator + > >> deadline I/O scheduler + SATA modules. > >> > >> v2.6.25 (not patched): CPU: 5.8330% (total) 7.5644% (I/O code only) > >> v2.6.25 + nomerges = 0: CPU: 5.8008% (total) 7.5806% (I/O code only) > >> v2.6.25 + nomerges = 1: CPU: 4.5404% (total) 5.9416% (I/O code only) > >> > >> v2.6.25 (not patched): DCM: 8.1967% (total) 10.5188% (I/O code only) > >> v2.6.25 + nomerges = 0: DCM: 7.2291% (total) 9.4087% (I/O code only) > >> v2.6.25 + nomerges = 1: DCM: 6.1989% (total) 8.0155% (I/O code only) > >> > >> I've typically been seeing a good 20-25% reduction in CPU samples, and > >> 10-15% in DCM samples for the random load w/ nomerges set to 1 compared > >> to set to 0 (looking at just the block code). > >> > >> [BTW: The I/O performance doesn't change much between the 3 sets of data > >> - the seek + I/O times themselves dominate things to such a large > >> extent. There is a very small improvement seen w/ nomerges=1, but <<1%.] > >> > >> It's not clear to me why 2.6.25 (not patched) requires /more/ cycles > >> than does the patched kernel w/ nomerges=0 -- it's been consistent in > >> the handful of runs I've done. I'm going to do a large set of runs for > >> each condition (not patched, nomerges=0 & nomerges=1) to verify that > >> this holds over multiple runs. I'm also going to check out sequential > >> loads to see what (if any) penalty the extra couple of checks incurs on > >> those (probably not noticeable). > >> > >> The first patch in the series adds the tunable; The second adds in the > >> check to skip the merge code; and the third adds in the check to skip > >> adding requests to hash lists for merging. > > > > The functionality is fine with me, merging is obviously a non-zero > > amount of cycles spent on IO and if you know it's in vain, may as well > > turn it off. One suggestion, though - if you add this as a performance > > rather than functionality change, I would suggest keeping the one-hit > > cache merge as that is essentially free. Better than free actually, > > since if you hit that merge point you'll be spending way less cycles > > than allocating+setting up a new request. > > > > Hi Jens - > > I'll look into retaining the one-hit cache merge functionality, remove > the errant elv_rqhas_del code, and repost w/ the results from the other > tests I've run.
Also please do a check where you only disable the front merge logic, as that is the most expensive bit (and the least likely to occur). I would not be surprised if just removing the front merge bit would get you the majority of the gain already. I have in the past considered just getting rid of that bit, as it rarely triggers and it is a costly rbtree lookup for each IO. The back merge lookup+merge should be cheaper, it's just a hash lookup.
-- Jens Axboe
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