(not sure why your mailer stripped me from the CC?) On Mon, Aug 03 2009, James Bottomley wrote: > On Mon, 2009-08-03 at 09:59 +0200, Jens Axboe wrote: > > On Mon, Aug 03 2009, Wu Fengguang wrote: > > > On Thu, Jul 30, 2009 at 08:06:49AM +0200, Jens Axboe wrote: > > > > On Wed, Jul 29 2009, Chris Mason wrote: > > > > > On Wed, Jul 29, 2009 at 11:18:45PM +0200, Jens Axboe wrote: > > > > > > On Wed, Jul 29 2009, Lars Ellenberg wrote: > > > > > > > I naively assumed, from the "readahead" in the name, that readahead > > > > > > > would be submitting READA bios. It does not. > > > > > > > > > > > > > > I recently did some statistics on how many READ and READA requests > > > > > > > we actually see on the block device level. > > > > > > > I was suprised that READA is basically only used for file system > > > > > > > internal meta data (and not even for all file systems), > > > > > > > but _never_ for file data. > > > > > > > > > > > > > > A simple > > > > > > > dd if=bigfile of=/dev/null bs=4k count=1 > > > > > > > will absolutely cause readahead of the configured amount, no problem. > > > > > > > But on the block device level, these are READ requests, where I'd > > > > > > > expected them to be READA requests, based on the name. > > > > > > > > > > > > > > This is because __do_page_cache_readahead() calls read_pages(), > > > > > > > which in turn is mapping->a_ops->readpages(), or, as fallback, > > > > > > > mapping->a_ops->readpage(). > > > > > > > > > > > > > > On that level, all variants end up submitting as READ. > > > > > > > > > > > > > > This may even be intentional. > > > > > > > But if so, I'd like to understand that. > > > > > > > > > > > > I don't think it's intentional, and if memory serves, we used to use > > > > > > READA when submitting read-ahead. Not sure how best to improve the > > > > > > situation, since (as you describe), we lose the read-ahead vs normal > > > > > > 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. > > Cancellation also came up several years ago with aio, which also has > requirements for it. Are they any real-world use cases for cancellation with aio? > > So I'm not sure it's a viable alternative, even if we restricted it to > > just tracking READA's, for instance. > > > > But I don't think we have any priority inversion to worry about, at > > least not from the CFQ perspective. > > The basic problem with cancellation when implemented at the storage > layer is that its an unusual operation. The storage primitives which > implement it aren't often invoked, so there's a lot of wariness to > implementing them in practice. For example, although SCSI has the abort > function, it's not implemented by a lot of controllers, so we'd have to > drop all pending I/O on the floor with a reset and then try and pick up > the pieces we wanted. Also, cancellation is racy since you never quite > know if the I/O hit the storage or not. > > On the back of this, we thought in 2003 or so that the best way to > implement cancellation was simply to do nothing and have something wait > around for the completion and throw it away. This has exactly the same > properties as storage implemented cancellation, but the benefit is that > we don't have to perturb the storage stack to do it. The approach I took (and exactly for the reasons you outline I think it's the only feasible one) is to allow cancellations of unstarted IO only. I can just imagine the pandora box we would open if we where to do this at the hardware/device level as well, so just chose not to go that far. -- Jens Axboe -- 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
