From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from verein.lst.de ([213.95.11.211]:51381 "EHLO newverein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754583AbcCCPLT (ORCPT ); Thu, 3 Mar 2016 10:11:19 -0500 Date: Thu, 3 Mar 2016 16:11:16 +0100 From: Christoph Hellwig To: Sagi Grimberg Cc: Christoph Hellwig , viro@zeniv.linux.org.uk, axboe@fb.com, milosz@adfin.com, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, linux-api@vger.kernel.org Subject: selective block polling and preadv2/pwritev2 revisited V3 Message-ID: <20160303151116.GA24614@lst.de> References: <1457017443-17662-1-git-send-email-hch@lst.de> <56D853B5.9000906@dev.mellanox.co.il> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <56D853B5.9000906@dev.mellanox.co.il> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, Mar 03, 2016 at 05:09:41PM +0200, Sagi Grimberg wrote: > Heh... Looks like you got your cover-letters mixed up :) Looks like it indeed.. Here is the right one: This series allows to selectively enable/disable polling for completions in the block layer on a per-I/O basis. For this it resurrects the preadv2/pwritev2 syscalls that Milosz prepared a while ago (and which are much simpler now due to VFS changes that happened in the meantime). That approach also had a man page update prepared, which I will resubmit with the current flags once this series makes it in. Polling for block I/O is important to reduce the latency on flash and post-flash storage technologies. On the fastest NVMe controller I have access to it almost halves latencies from over 7 microseconds to about 4 microseonds. But it only is usesful if we actually care for the latency of this particular I/O, and generally is a waste if enabled for all I/O to a given device. This series uses the per-I/O flags in preadv2/pwritev2 to control this behavior. The alternative would be a new O_* flag set at open time or using fcntl, but this is still to corse-grained for some applications and we're starting to run out out of open flags. Note that there are plenty of other use cases for preadv2/pwritev2 as well, but I'd like to concentrate on this one for now. Example are: non-blocking reads (the original purpose), per-I/O O_SYNC, user space support for T10 DIF/DIX applications tags and probably some more. Changes since V2: - minor style fixes - various changelog updates - dropped the unused REQ_POLL flag Changes since V1: - rebased on top of Linux 4.5-rc5