On Thu, Aug 21, 2014 at 11:13 AM, Ming Lei <[email protected]> wrote: > On Thu, Aug 21, 2014 at 10:58 AM, Jens Axboe <[email protected]> wrote: >> On 2014-08-20 21:54, Ming Lei wrote: >>>>> >>>>> From my investigation, context switch increases almost 50% with >>>>> workqueue compared with kthread in loop in a quad-core VM. With >>>>> kthread, requests may be handled as batch in cases which won't be >>>>> blocked in read()/write()(like null_blk, tmpfs, ...), but it is >>>>> impossible >>>>> with >>>>> workqueue any more. Also block plug&unplug should have been used >>>>> with kthread to optimize the case, especially when kernel AIO is >>>>> applied, >>>>> still impossible with work queue too. >>>> >>>> >>>> >>>> OK, that one is actually a good point, since one need not do per-item >>>> queueing. We could handle different units, though. And we should have >>>> proper >>>> marking of the last item in a chain of stuff, so we might even be able to >>>> offload based on that instead of doing single items. It wont help the >>>> sync >>>> case, but for that, workqueue and kthread would be identical. >>> >>> >>> We may do that by introducing callback of queue_rq_list in blk_mq_ops, >>> and I will figure out one patch today to see if it can help the case. >> >> >> I don't think we should add to the interface, I prefer keeping it clean like >> it is right now. At least not if we can get around it. My point is that the >> driver already knows when the chain is complete, when REQ_LAST is set. So >> before that event triggers, it need not kick off IO, or at least i could do >> it in batches before that. That may not be fully reliable in case of >> queueing errors, but if REQ_LAST or 'error return' is used as the way to >> kick off pending IO, then that should be good enough. Haven't audited this >> in a while, but at least that is the intent of REQ_LAST. > > Yes, I thought of too, but driver need another context for handling that, > either workqueue or kthread, which may cause the introduced per-device > workqueue useless.
Hmmm, a list should be enough, will do that. Thanks, -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
