Messages in this thread | | | Date | Thu, 24 Apr 2008 09:31:50 -0400 | From | "Alan D. Brunelle" <> | Subject | Re: [RFC][PATCH 0/3] Skip I/O merges when disabled |
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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. > > Alan D. Brunelle > Hewlett-Packard
The results over 25 runs (10-minutes each) look good, as noted yesterday, /very/ slightly better I/Os per seconds with nomerges=1:
Kernel NM I/Os per second -------------------- -- --------------- 2.6.25 (not patched) 483,727.36 2.6.25 + nomerges 0 483,880.96 2.6.25 + nomerges 1 483,921.92
The CPU and DCM samples in the block I/O code again were better w/ nomerges=1 averaged over the 25 runs (about 23.8% fewer cycles needed to do the work in the block I/O code):
v2.6.25 (not patched): CPU: 5.779% (total) 7.544% (I/O code only) v2.6.25 + nomerges = 0: CPU: 5.496% (total) 7.199% (I/O code only) v2.6.25 + nomerges = 1: CPU: 4.403% (total) 5.771% (I/O code only)
v2.6.26 (not patched): DCM: 7.986% (total) 10.246% (I/O code only) v2.6.25 + nomerges = 0: DCM: 8.213% (total) 10.514% (I/O code only) v2.6.25 + nomerges = 1: DCM: 6.670% (total) 8.525% (I/O code only)
Alan
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