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Date: Thu, 10 Sep 2009 11:02:56 +1000 From: Con Kolivas <kernel@...ivas.org> To: Jens Axboe <jens.axboe@...cle.com> Cc: Nikos Chantziaras <realnc@...or.de>, Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-kernel@...r.kernel.org Subject: Re: BFS vs. mainline scheduler benchmarks and measurements On Thu, 10 Sep 2009 06:50:43 Jens Axboe wrote: > On Wed, Sep 09 2009, Nikos Chantziaras wrote: > > On 09/09/2009 09:04 PM, Ingo Molnar wrote: > >> [...] > >> > >> * Jens Axboe<jens.axboe@...cle.com> wrote: > >>> On Wed, Sep 09 2009, Jens Axboe wrote: > >>> [...] > >>> BFS210 runs on the laptop (dual core intel core duo). With make -j4 > >>> running, I clock the following latt -c8 'sleep 10' latencies: > >>> > >>> -rc9 > >>> > >>> Max 17895 usec > >>> Avg 8028 usec > >>> Stdev 5948 usec > >>> Stdev mean 405 usec > >>> > >>> Max 17896 usec > >>> Avg 4951 usec > >>> Stdev 6278 usec > >>> Stdev mean 427 usec > >>> > >>> Max 17885 usec > >>> Avg 5526 usec > >>> Stdev 6819 usec > >>> Stdev mean 464 usec > >>> > >>> -rc9 + mike > >>> > >>> Max 6061 usec > >>> Avg 3797 usec > >>> Stdev 1726 usec > >>> Stdev mean 117 usec > >>> > >>> Max 5122 usec > >>> Avg 3958 usec > >>> Stdev 1697 usec > >>> Stdev mean 115 usec > >>> > >>> Max 6691 usec > >>> Avg 2130 usec > >>> Stdev 2165 usec > >>> Stdev mean 147 usec > >> > >> At least in my tests these latencies were mainly due to a bug in > >> latt.c - i've attached the fixed version. > >> > >> The other reason was wakeup batching. If you do this: > >> > >> echo 0> /proc/sys/kernel/sched_wakeup_granularity_ns > >> > >> ... then you can switch on insta-wakeups on -tip too. > >> > >> With a dual-core box and a make -j4 background job running, on > >> latest -tip i get the following latencies: > >> > >> $ ./latt -c8 sleep 30 > >> Entries: 656 (clients=8) > >> > >> Averages: > >> ------------------------------ > >> Max 158 usec > >> Avg 12 usec > >> Stdev 10 usec > > > > With your version of latt.c, I get these results with 2.6-tip vs > > 2.6.31-rc9-bfs: > > > > > > (mainline) > > Averages: > > ------------------------------ > > Max 50 usec > > Avg 12 usec > > Stdev 3 usec > > > > > > (BFS) > > Averages: > > ------------------------------ > > Max 474 usec > > Avg 11 usec > > Stdev 16 usec > > > > > > However, the interactivity problems still remain. Does that mean it's > > not a latency issue? > > It probably just means that latt isn't a good measure of the problem. > Which isn't really too much of a surprise. And that's a real shame because this was one of the first real good attempts I've seen to actually measure the difference, and I thank you for your efforts Jens. I believe the reason it's limited is because all you're measuring is time from wakeup and the test app isn't actually doing any work. The issue is more than just waking up as fast as possible, it's then doing some meaningful amount of work within a reasonable time frame as well. What the "meaningful amount of work" and "reasonable time frame" are, remains a mystery, but I guess could be added on to this testing app. What does please me now, though, is that this message thread is finally concentrating on what BFS was all about. The fact that it doesn't scale is no mystery whatsoever. The fact that that throughput and lack of scaling was what was given attention was missing the point entirely. To point that out I used the bluntest response possible, because I know that works on lkml (does it not?). Unfortunately I was so blunt that I ended up writing it in another language; Troll. So for that, I apologise. The unfortunate part is that BFS is still far from a working, complete state, yet word got out that I had "released" something, which I had not, but obviously there's no great distinction between putting something on a server for testing, and a real release with an announce. BFS is a scheduling experiment to demonstrate what effect the cpu scheduler really has on the desktop and how it might be able to perform if we design the scheduler for that one purpose. It pleases me immensely to see that it has already spurred on a flood of changes to the interactivity side of mainline development in its few days of existence, including some ideas that BFS uses itself. That in itself, to me, means it has already started to accomplish its goal, which ultimately, one way or another, is to improve what the CPU scheduler can do for the linux desktop. I can't track all the sensitive areas of the mainline kernel scheduler changes without getting involved more deeply than I care to so it would be counterproductive of me to try and hack on mainline. I much prefer the quieter inbox. If people want to use BFS for their own purposes or projects, or even better help hack on it, that would make me happy for different reasons. I will continue to work on my little project -in my own time- and hope that it continues to drive further development of the mainline kernel in its own way. We need more experiments like this to question what we currently have and accept. Other major kernel subsystems are no exception. Regards, -- -ck <code before rhetoric> -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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