On Sun, Dec 16, 2012 at 03:15:49PM +1100, Dave Chinner wrote:

On Sun, Dec 16, 2012 at 03:35:49AM +0000, Eric Wong wrote:

quoted

Dave Chinner [off-list ref] wrote:

quoted

On Sun, Dec 16, 2012 at 12:25:49AM +0000, Eric Wong wrote:

quoted

Alan Cox [off-list ref] wrote:

quoted

On Sat, 15 Dec 2012 00:54:48 +0000
Eric Wong [off-list ref] wrote:

quoted

Applications streaming large files may want to reduce disk spinups and
I/O latency by performing large amounts of readahead up front

quoted

This could also be a use case for an audio/video player.

Sure, but this can all be handled by a userspace application. If you
want to avoid/batch IO to enable longer spindown times, then you
have to load the file into RAM somewhere, and you don't need special
kernel support for that.

quoted

So no, there's no difference that matters between the approaches.
But I think doing this in the kernel is easier for userspace users.

The kernel provides mechanisms for applications to use. You have not
mentioned anything new that requires a new kernel mechanism to
acheive - you just need to have the knowledge to put the pieces
together properly.  People have been solving this same problem for
the last 20 years without needing to tweak fadvise(). Or even having
an fadvise() syscall...

Nothing about low latency IO or streaming IO is simple or easy, and
changing how readahead works doesn't change that fact. All it does
is change the behaviour of every other application that uses
fadvise() to minimise IO latency....

Hi Dave,

I am wondering this patch might be a good idea to reduce the latency of
fadvise() syscall itself.  I do a really simple test in my desktop to
measure the latency of fadvise syscall.  Before applying this patch,
fadvise syscall takes 32 microseconds.  After applying the patch, it
only takes 4 microseconds. (I was surprised that it takes a very long
time!)

Actually we observe a latency after using fadvise.  But I don't find a
proper time to look at this problem.  So I guess this patch might be
useful to reduce latency.

Regards,
                                                - Zheng

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