From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pb0-f47.google.com (mail-pb0-f47.google.com [209.85.160.47]) by kanga.kvack.org (Postfix) with ESMTP id 1DD266B00C4 for ; Thu, 17 Oct 2013 16:23:53 -0400 (EDT) Received: by mail-pb0-f47.google.com with SMTP id rr4so2778533pbb.20 for ; Thu, 17 Oct 2013 13:23:52 -0700 (PDT) Date: Thu, 17 Oct 2013 13:23:48 -0700 From: Andrew Morton Subject: Re: [PATCH] readahead: fix sequential read cache miss detection Message-Id: <20131017132348.a89c6cb5222eda83fb0ce079@linux-foundation.org> In-Reply-To: <1382033352-21225-1-git-send-email-damien.ramonda@intel.com> References: <1382033352-21225-1-git-send-email-damien.ramonda@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Damien Ramonda Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, pierre.tardy@intel.com, fengguang.wu@intel.com, david.a.cohen@intel.com On Thu, 17 Oct 2013 20:09:12 +0200 Damien Ramonda wrote: > The kernel's readahead algorithm sometimes interprets random read > accesses as sequential and triggers unnecessary data prefecthing > from storage device (impacting random read average latency). > > In order to identify sequential cache read misses, the readahead > algorithm intends to check whether offset - previous offset == 1 > (trivial sequential reads) or offset - previous offset == 0 > (sequential reads not aligned on page boundary): > > if (offset - (ra->prev_pos >> PAGE_CACHE_SHIFT) <= 1UL) > > The current offset is stored in the "offset" variable of type > "pgoff_t" (unsigned long), while previous offset is stored in > "ra->prev_pos" of type "loff_t" (long long). Therefore, > operands of the if statement are implicitly converted to type > long long. Consequently, when previous offset > current offset > (which happens on random pattern), the if condition is true > and access is wrongly interpeted as sequential. An unnecessary > data prefetching is triggered, impacting the average > random read latency. > > Storing the previous offset value in a "pgoff_t" variable > (unsigned long) fixes the sequential read detection logic. Do you have any performance testing results which would permit people to understand the significance of this change? Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org