Re: Why does __do_page_cache_readahead submit READ, not READA?

[Wu Fengguang <fengguang.wu@intel.com>]

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