On 02/28/2013 04:04 PM, Mike Galbraith wrote: > On Thu, 2013-02-28 at 15:40 +0800, Michael Wang wrote: >> Hi, Mike >> >> Thanks for your reply. >> >> On 02/28/2013 03:18 PM, Mike Galbraith wrote: >>> On Thu, 2013-02-28 at 14:38 +0800, Michael Wang wrote: >>> >>>> + /* >>>> + * current is the only task on rq and it is >>>> + * going to sleep, current cpu will be a nice >>>> + * candidate for p to run on. >>>> + */ >>> >>> The sync hint only means it might be going to sleep soon, and even then, >>> there can still be enough execution overlap to be a win to schedule >>> cross core. Sched pipe numbers will always be much prettier if you do >>> wakeup cpu affine, as it's ~100% scheduler and ~100% sync. >> >> Hmm.. so it's the comparison between 'cache benefit - execution overlap' >> and 'latency - execution overlap'? > > Yeah. You'll always lose power cross core, and throughput breakeven and > win depends on convertible overlap, and how much L2 miss etc costs. For > sched pipe there is no win, but for other sync hint users there is. > >> I could not estimate how many latency will be added to wait for current >> going to sleep (it should be faster than access cold data, isn't it?), >> but I really like the cache benefit, unless sync doesn't means current >> is going to sleep every time, but that's the promise of WF_SYNC, isn't it? > > It would be nice if it _were_ a promise, but it is not, it's a hint.
Bad to know :( Should we fix it or this is by designed? The comments after WF_SYNC cheated me... Regards, Michael Wang > >> You may lose >>> a lot on other stuff if you interpret the hint as gospel truth. >> >> Could you please give more details on this point? > > tbench, mysql+oltp, on and on use the sync hint, many things jabber on > localhost, use the sync hint, and have been shown in cold hard numbers > to benefit, some things massively from cross core scheduling. You lose > for sure at extreme context rates, but it has to be pretty darn high to > be a guaranteed loser. > > That's why select_idle_sibling() is so very damn annoying. >>> IMHO, sched pipe is a "how fat have I become" benchmark, not "how well >>> do I perform". The scheduler performs well when it makes more work >>> happen. Playing ping-pong with yourself is _exercise_, not a job :) >> >> That's right, may be I'm using the wrong description, it's the ops/sec >> which has been doubled, that means 'fat', correct? > > In this case, it means you're not running a kernel with nohz on a chain, > running two schedulers is more expensive than running one, and missing > L2 each and every time hurts very badly when the load is ultra skinny. > > -Mike > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
