linux-kernel - random AIO reads (do_generic_mapping

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Message-ID: <464125BC.109@cs.binghamton.edu>
Date:	Tue, 08 May 2007 21:37:00 -0400
From:	"Michael R. Hines" <mhines@...binghamton.edu>
To:	linux-kernel@...r.kernel.org
CC:	mhines@...binghamton.edu
Subject: random AIO reads (do_generic_mapping_read)

Howdy (please CC replies directly),

I have a very specific question regarding the behavior of
do_generic_mapping_read().

Here's my 2.6.20 setup: I have a stable block device driver that has
random-access style latencies (around 80 microseconds. zero seek-time).

From userland: I have a simple AIO io_submit/io_suspend
program that repeatedly fires off *hundreds random AIO reads*

Here's what I would like to happen:

Since my driver does not seek (i.e. I/O not helped by mm/readahead.c)
I would like my AIOs to be *overlapped* when sent to my driver.
Currently, do_generic_mapping_read() will block on every read,
one-by-one. Instead I want the bio's to be fired off to my driver as
fast as possible without calling lock_page(). Rather the read should
return and allow io_submit_one to fire off more reads without waiting
for the userspace-copy - it would instead happen later in the future.

Furthermore: I *do not* want to avoid the page cache (which temporarily
solves the problem). I still want the support of the page-cache, I just
instead need those I/Os to not block or sleep while waiting for a single
I/O to complete.

Obviously if this were a disk, such a requirement would be very stupid
(and useless). But without it, my low-latency driver is under-utilized.

Is this possible right now?

-- 
/**************************************/
Michael R. Hines
Grad Student, SUNY Binghamton
http://www.cs.binghamton.edu/~mhines
/ *************************************/

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