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SubjectRe: [RFC] IO scheduler based IO controller V9
Vivek Goyal wrote:
> On Thu, Sep 10, 2009 at 05:18:25PM +0200, Jerome Marchand wrote:
>> Vivek Goyal wrote:
>>> Hi All,
>>>
>>> Here is the V9 of the IO controller patches generated on top of 2.6.31-rc7.
>>
>> Hi Vivek,
>>
>> I've run some postgresql benchmarks for io-controller. Tests have been
>> made with 2.6.31-rc6 kernel, without io-controller patches (when
>> relevant) and with io-controller v8 and v9 patches.
>> I set up two instances of the TPC-H database, each running in their
>> own io-cgroup. I ran two clients to these databases and tested on each
>> that simple request:
>> $ select count(*) from LINEITEM;
>> where LINEITEM is the biggest table of TPC-H (6001215 entries,
>> 720MB). That request generates a steady stream of IOs.
>>
>> Time is measure by psql (\timing switched on). Each test is run twice
>> or more if there is any significant difference between the first two
>> runs. Before each run, the cache is flush:
>> $ echo 3 > /proc/sys/vm/drop_caches
>>
>>
>> Results with 2 groups of same io policy (BE) and same io weight (1000):
>>
>> w/o io-scheduler io-scheduler v8 io-scheduler v9
>> first second first second first second
>> DB DB DB DB DB DB
>>
>> CFQ 48.4s 48.4s 48.2s 48.2s 48.1s 48.5s
>> Noop 138.0s 138.0s 48.3s 48.4s 48.5s 48.8s
>> AS 46.3s 47.0s 48.5s 48.7s 48.3s 48.5s
>> Deadl. 137.1s 137.1s 48.2s 48.3s 48.3s 48.5s
>>
>> As you can see, there is no significant difference for CFQ
>> scheduler. There is big improvement for noop and deadline schedulers
>> (why is that happening?). The performance with anticipatory scheduler
>> is a bit lower (~4%).
>>
>
> Ok, I think what's happening here is that by default slice lenght for
> a queue is 100ms. When you put two instances of DB in two different
> groups, one streaming reader can run at max for 100ms at a go and then
> we switch to next reader.
>
> But when both the readers are in root group, then AS lets run one reader
> to run at max 250ms (sometimes 125ms and sometimes 250ms based on at what
> time as_fifo_expired() was invoked).
>
> So because a reader gets to run longer at one stretch in root group, it
> reduces number of seeks and leads to little enhanced throughput.
>
> If you change the /sys/block/<disk>/queue/iosched/slice_sync to 250 ms, then
> one group queue can run at max for 250ms before we switch the queue. In
> this case you should be able to get same performance as in root group.
>
> Thanks
> Vivek

Indeed. When I run the benchmark with slice_sync = 250ms, I get results
close to the one for both instances running within the root group:
first group 46.1s and second group 46.4s.

Jerome




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