[Devel] Re: IO scheduler based IO controller V10

[Jens Axboe]

On Fri, Oct 02 2009, Mike Galbraith wrote:
> On Fri, 2009-10-02 at 19:37 +0200, Jens Axboe wrote:
> > On Fri, Oct 02 2009, Ingo Molnar wrote:
> > > 
> > > * Jens Axboe <jens.axboe at oracle.com> wrote:
> > > 
> > > > On Fri, Oct 02 2009, Ingo Molnar wrote:
> > > > > 
> > > > > * Jens Axboe <jens.axboe at oracle.com> wrote:
> > > > > 
> > > > > > It's not _that_ easy, it depends a lot on the access patterns. A 
> > > > > > good example of that is actually the idling that we already do. 
> > > > > > Say you have two applications, each starting up. If you start them 
> > > > > > both at the same time and just care for the dumb low latency, then 
> > > > > > you'll do one IO from each of them in turn. Latency will be good, 
> > > > > > but throughput will be aweful. And this means that in 20s they are 
> > > > > > both started, while with the slice idling and priority disk access 
> > > > > > that CFQ does, you'd hopefully have both up and running in 2s.
> > > > > > 
> > > > > > So latency is good, definitely, but sometimes you have to worry 
> > > > > > about the bigger picture too. Latency is more than single IOs, 
> > > > > > it's often for complete operation which may involve lots of IOs. 
> > > > > > Single IO latency is a benchmark thing, it's not a real life 
> > > > > > issue. And that's where it becomes complex and not so black and 
> > > > > > white. Mike's test is a really good example of that.
> > > > > 
> > > > > To the extent of you arguing that Mike's test is artificial (i'm not 
> > > > > sure you are arguing that) - Mike certainly did not do an artificial 
> > > > > test - he tested 'konsole' cache-cold startup latency, such as:
> > > > 
> > > > [snip]
> > > > 
> > > > I was saying the exact opposite, that Mike's test is a good example of 
> > > > a valid test. It's not measuring single IO latencies, it's doing a 
> > > > sequence of valid events and looking at the latency for those. It's 
> > > > benchmarking the bigger picture, not a microbenchmark.
> > > 
> > > Good, so we are in violent agreement :-)
> > 
> > Yes, perhaps that last sentence didn't provide enough evidence of which
> > category I put Mike's test into :-)
> > 
> > So to kick things off, I added an 'interactive' knob to CFQ and
> > defaulted it to on, along with re-enabling slice idling for hardware
> > that does tagged command queuing. This is almost completely identical to
> > what Vivek Goyal originally posted, it's just combined into one and uses
> > the term 'interactive' instead of 'fairness'. I think the former is a
> > better umbrella under which to add further tweaks that may sacrifice
> > throughput slightly, in the quest for better latency.
> > 
> > It's queued up in the for-linus branch.
> 
> FWIW, I did a matrix of Vivek's patch combined with my hack.  Seems we
> do lose a bit of dd throughput over stock with either or both.
> 
> dd pre         65.1     65.4     67.5     64.8     65.1   65.5     fairness=1 overload_delay=1
> perf stat      1.70     1.94     1.32     1.89     1.87    1.7
> dd post        69.4     62.3     69.7     70.3     69.6   68.2
> 
> dd pre         67.0     67.8     64.7     64.7     64.9   65.8     fairness=1 overload_delay=0
> perf stat      4.89     3.13     2.98     2.71     2.17    3.1
> dd post        67.2     63.3     62.6     62.8     63.1   63.8
> 
> dd pre         65.0     66.0     66.9     64.6     67.0   65.9     fairness=0 overload_delay=1
> perf stat      4.66     3.81     4.23     2.98     4.23    3.9
> dd post        62.0     60.8     62.4     61.4     62.2   61.7
> 
> dd pre         65.3     65.6     64.9     69.5     65.8   66.2     fairness=0 overload_delay=0
> perf stat     14.79     9.11    14.16     8.44    13.67   12.0
> dd post        64.1     66.5     64.0     66.5     64.4   65.1

I'm not too worried about the "single IO producer" scenarios, and it
looks like (from a quick look) that most of your numbers are within some
expected noise levels. It's the more complex mixes that are likely to
cause a bit of a stink, but lets worry about that later. One quick thing
would be to read eg 2 or more files sequentially from disk and see how
that performs.

If you could do a cleaned up version of your overload patch based on
this:

http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=1d2235152dc745c6d94bedb550fea84cffdbf768

then lets take it from there.

-- 
Jens Axboe

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