From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ric Wheeler Subject: high resolution timers, scheduling & sleep granularity Date: Fri, 01 Aug 2008 08:05:37 -0400 Message-ID: <4892FC11.4020105@redhat.com> Reply-To: rwheeler@redhat.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit To: Thomas Gleixner , Ingo Molnar , Josef Bacik , linux-fsdevel@vger.kernel.org, Chris Mason , linux-kernel@v Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Hi Thomas & Ingo, Josef has been working on some patches to try and get ext3/4 to dynamically detect the latency of a storage device and use that base latency to tune the amount of time we sleep waiting for others to join in a transaction. The logic in question lives in jbd/transaction.c (transaction_stop). The code was originally developed to try and allow multiple threads to join in a big, slow transaction. For example, transacations that write to a slow ATA or S-ATA drive take in the neighborhood of 10 to 20 ms. Faster devices, for example a disk array, can complete the transaction in 1.3 ms. Even higher speed SSD devices boast of a latency of 0.1ms, not to mention RAM disks ;-) The current logic makes us wait way too long, especially with a 250HZ kernel since we sleep many times longer than it takes to complete the IO ;-) Do either of you have any thoughts on how to get a better, fine grained sleep capability that we could use that would allow us to sleep in sub-jiffie chunks? Regards, Ric