On Tue, Jun 24 2008, Nick Piggin wrote:

On Tuesday 24 June 2008 21:36, Miklos Szeredi wrote:

quoted

quoted

It's an unfortunate side effect of the read-ahead, I'd much rather just
get rid of that. It _should_ behave like the non-ra case, when a page is
added it merely has IO started on it. So we want to have that be
something like

        if (!PageUptodate(page) && !PageInFlight(page))
                ...

basically like PageWriteback(), but for read-in.

OK it could be done, possibly at great pain.  But why is it important?

It has been considered, but adding atomic operations on these paths
always really hurts. Adding something like this would basically be
another at least 2 atomic operations that can never be removed again...

Provided that you've done the sync readahead earlier, it presumably
should be a very rare case to have to start new IO in the loop
below, right? In which case, I wonder if we couldn't move that 2nd
loop out of generic_file_splice_read and into
page_cache_pipe_buf_confirm. 

That's a good point, moving those blocks of code to the other end makes
a lot of sense. Or just kill the read-ahead, or at least do it
differently. It's definitely an oversight/bug having splice from file
block on the pages it just issued read-ahead for.

quoted

What's the use case where it matters that splice-in should not block
on the read?

It just makes it generally less able to pipeline IO and computation,
doesn't it?

Precisely!

quoted

And note, after the pipe is full it will block no matter what, since
the consumer will have to wait until the page is brought uptodate, and
can only then commence with getting the data out from the pipe.

True, but (especially with patches to variably size the pipe buffer)
I imagine programs could be designed fairly carefully to the size of
the buffer (and not just things that blast bulk data down the pipe...)

Yep, that's the whole premise for the dynpipe branch I've been carrying
around for some time.

-- 
Jens Axboe

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