Re: Why does __do_page_cache_readahead submit READ, not READA?

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

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 ;)

Thanks,
Fengguang

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