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Date: Mon, 25 Apr 2011 09:01:59 +0800 From: Shaohua Li <shaohua.li@...el.com> To: Tejun Heo <htejun@...il.com> Cc: Jens Axboe <jaxboe@...ionio.com>, lkml <linux-kernel@...r.kernel.org>, "Shi, Alex" <alex.shi@...el.com>, "Chen, Tim C" <tim.c.chen@...el.com> Subject: Re: [RFC]block: add flush request at head On Sat, 2011-04-23 at 06:57 +0800, Tejun Heo wrote: > Hello, > > On Mon, Apr 18, 2011 at 04:25:57PM +0800, Shaohua Li wrote: > > then why requeue adds request at head? we could have the similar issue. > > SCSI doesn't seem to do it anymore but it used to cache scmd at > rq->special over requeues so that it doesn't have to re-initialize > requests across requeues, which means that unprepped request getting > ahead of requeued ones may lead to deadlock due to resource > starvation, so that's why requeue uses front queueing. > > The code changed over time and the above requirement might not be > necessary at this point. I don't know. However, block layer doesn't > have any method to enforce that requests can't hold any extra resource > on requeue and having such difficult to trigger deadlock condition > dormant is scary. > > What kind of benchmarking are we talking about on which kernel? > blk-flush support has been revamped twice recently. 2.6.38 stripped > out the block layer barrier thing and then it got re-reimplemented for > 2.6.39 to support advanced flush merging. If the regression (for > which benchmark btw?) was visible on the older reimplementation, I'd > really like to know how it behaves on 2.6.39-rcX. > > If the problem is localized to 2.6.38, oh well, too bad, but I don't > think we care too much. If some distro is basing their kernel on > 2.6.38 and the flush regression is hurting them, backporting the new > implementation from 2.6.39 shouldn't be too difficult after all. The > reimplementation was almost self-contained. > > If the regression affects 2.6.39 implementation too, eh well, we need > to think of something, but I'd really like to know what kind of > workload we're talking about. > > > I'll look at this. Optimizing this one should fix the regression too. On > > the other hand, adding flush request at head if it just follows a flush > > still has its advantage, because drive cache is already flushed out. > > New implementation wouldn't issue two flushes back to back like that, > it doesn't make any sense to begin with. Again, what have you been > testing and how? Hi Tejun, this is a regression from 2.6.39-rc2 compared to 2.6.39-rc1, so this isn't related to the flush rewritten. Workload is sysbench fileio, please see the first mail at the thread for detail. Thanks, Shaohua -- 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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