From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 20 Aug 2002 16:52:35 +0100 From: "Stephen C. Tweedie" Subject: Re: active_mm and mm Message-ID: <20020820165235.I2645@redhat.com> References: <20020820101950.A2645@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: ; from haih@eecs.umich.edu on Tue, Aug 20, 2002 at 10:55:04AM -0400 Sender: owner-linux-mm@kvack.org Return-Path: To: Hai Huang Cc: "Stephen C. Tweedie" , linux-mm@kvack.org List-ID: Hi, On Tue, Aug 20, 2002 at 10:55:04AM -0400, Hai Huang wrote: > Ok, I see why we're differentiating between mm and active_mm, but is this > actually giving us a lot of benefits considering the number of context switches > that would actually take advantage of this feature is probably small > (well, it depends on the workload). It's actually enormous. There are a lot of kernel daemons that do background IO, for example. Those are often waking up after an IO completes, doing a tiny amount of work to submit new IO, then sleeping again. Even more significant in many workloads is the idle task. > Also, is the tlb flush operation that > expensive? Yes. Modern cpus are _way_ faster than main memory, and they rely utterly on the cache architecture to keep them busy. Doing a tlb flush forces the CPU to go back to main memory up to 2 times for every single address translation that follows until the tlb is full again. That's an enormous cost, especially on rapidly-switching workloads. --Stephen -- 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/