Re: RFC: Allow block drivers to poll for I/O instead of sleeping
From: Bart Van Assche
Date: Tue Jun 25 2013 - 03:07:42 EST
On 06/25/13 05:18, Matthew Wilcox wrote:
On Mon, Jun 24, 2013 at 10:07:51AM +0200, Ingo Molnar wrote:
I'm wondering, how will this scheme work if the IO completion latency is a
lot more than the 5 usecs in the testcase? What if it takes 20 usecs or
100 usecs or more?
There's clearly a threshold at which it stops making sense, and our
current NAND-based SSDs are almost certainly on the wrong side of that
threshold! I can't wait for one of the "post-NAND" technologies to make
it to market in some form that makes it economical to use in an SSD.
The problem is that some of the people who are looking at those
technologies are crazy. They want to "bypass the kernel" and "do user
space I/O" because "the kernel is too slow". This patch is part of an
effort to show them how crazy they are. And even if it doesn't convince
them, at least users who refuse to rewrite their applications to take
advantage of magical userspace I/O libraries will see real performance
benefits.
Recently I attended an interesting talk about this subject in which it
was proposed not only to bypass the kernel for access to high-IOPS
devices but also to allow byte-addressability for block devices. The
slides that accompanied that talk can be found here (includes a
performance comparison with the traditional block driver API):
Bernard Metzler, On Suitability of High-Performance Networking API for
Storage, OFA Int'l Developer Workshop, April 24, 2013
(http://www.openfabrics.org/ofa-documents/presentations/doc_download/559-on-suitability-of-high-performance-networking-api-for-storage.html).
This approach leaves the choice of whether to use polling or an
interrupt-based completion notification to the user of the new API,
something the Linux InfiniBand RDMA verbs API already allows today.
Bart.
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