Re: [RFC][PATCH 0/3] Skip I/O merges when disabled

["Alan D. Brunelle" <Alan.Brunelle@hp.com>]

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.

Thanks,
Alan