On Mon, Aug 03, 2009 at 05:37:53PM +0800, Jens Axboe wrote: > On Mon, Aug 03 2009, Wu Fengguang wrote: > > On Mon, Aug 03, 2009 at 05:25:15PM +0800, Jens Axboe wrote: > > > On Mon, Aug 03 2009, Wu Fengguang wrote: > > > > On Mon, Aug 03, 2009 at 03:59:33PM +0800, Jens Axboe wrote: > > > > > On Mon, Aug 03 2009, Wu Fengguang wrote: > > > > > > On Thu, Jul 30, 2009 at 08:06:49AM +0200, Jens Axboe wrote: > > > > > > > > > read at that level. I did some experimentation some time ago for > > > > > > > > > flagging this, see: > > > > > > > > > > > > > > > > > > http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff;h=16cfe64e3568cda412b3cf6b7b891331946b595e > > > > > > > > > > > > > > > > > > which should pass down READA properly. > > > > > > > > > > > > > > > > One of the problems in the past was that reada would fail if there > > > > > > > > wasn't a free request when we actually wanted it to go ahead and wait. > > > > > > > > Or something. We've switched it around a few times I think. > > > > > > > > > > > > > > Yes, we did used to do that, whether it was 2.2 or 2.4 I > > > > > > > don't recall :-) > > > > > > > > > > > > > > It should be safe to enable know, whether there's a prettier way > > > > > > > than the above, I don't know. It works by detecting the read-ahead > > > > > > > marker, but it's a bit of a fragile design. > > > > > > > > > > > > Another consideration is io-priority reversion and the overheads > > > > > > required to avoid it: > > > > > > > > > > > > readahead(pages A-Z) => READA IO for pages A-Z > > > > > > <short time later> > > > > > > read(page A) => blocked => find the request that contains page A > > > > > > and requeue/kick it as READ IO > > > > > > > > > > > > The page-to-request lookups are not always required but nevertheless > > > > > > the complexity and overheads won't be trivial. > > > > > > > > > > > > The page-to-request lookup feature would be also useful for "advanced" > > > > > > features like io-canceling (if implemented, hwpoison could be its > > > > > > first user ;) > > > > > > > > > > I added that 3-4 years ago or so, to experiment with in-kernel > > > > > cancellation for things like truncate(). Tracking pages is not cheap, > > > > > and since the write cancelling wasn't really very sucessful, I didn't go > > > > > ahead with it. > > > > > > > > Ah OK. > > > > > > > > > So I'm not sure it's a viable alternative, even if we restricted it to > > > > > just tracking READA's, for instance. > > > > > > > > Kind of agreed. I guess it won't benefit too much workloads to default > > > > to READA; for most workloads it would be pure overheads if considering > > > > priority inversion. > > > > > > > > > But I don't think we have any priority inversion to worry about, at > > > > > least not from the CFQ perspective. > > > > > > > > The priority inversion problem showed up in an early attempt to do > > > > boot time prefetching. I guess this problem was somehow circumvented > > > > by limiting the prefetch depth and do prefetches in original read > > > > order instead of disk location order (Arjan cc'ed). > > > > > > But was that not due to the prefetcher running at a lower cpu priority? > > > > Yes, it is. Thus the priority inversion problem. > > > > > Just flagging a reada hint will not change your priority in the IO > > > scheduler, so we should have no priority inversion there. > > > > Ah OK. So READA merely means "don't try hard on error" for now. > > Sorry I implicitly associated it with some priority class.. > > Well not necessarily, it could also have some priority implications in > the scheduler. My point is just that it need not be severe enough to > introduce priority inversions, so that we need a specific tracking > framework to graduate READA to READ. Right, that's a good point. Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
