From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933197AbcBCQTj (ORCPT ); Wed, 3 Feb 2016 11:19:39 -0500 Received: from gum.cmpxchg.org ([85.214.110.215]:56496 "EHLO gum.cmpxchg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751901AbcBCQTi (ORCPT ); Wed, 3 Feb 2016 11:19:38 -0500 Date: Wed, 3 Feb 2016 11:19:10 -0500 From: Johannes Weiner To: Martijn Coenen Cc: linux-mm@kvack.org, Anton Vorontsov , Andrew Morton , Michal Hocko , Vladimir Davydov , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Jonathan Corbet Subject: Re: [PATCH] mm: vmpressure: make vmpressure_window a tunable. Message-ID: <20160203161910.GA10440@cmpxchg.org> References: <001a114b360c7fdb9b052adb91d6@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <001a114b360c7fdb9b052adb91d6@google.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Feb 03, 2016 at 11:06:20AM +0100, Martijn Coenen wrote: > The window size used for calculating vm pressure > events was previously fixed at 512 pages. The > window size has a big impact on the rate of notifications > sent off to userspace, in particular when using the > "low" level. On machines with a lot of memory, the > current value may be excessive. > > On the other hand, making the window size depend on > machine size does not allow userspace to change the > notification rate based on the current state of the > system. For example, when a lot of memory is still > available, userspace may want to increase the window > since it's not interested in receiving notifications > for every 2MB scanned. > > This patch makes vmpressure_window a sysctl tunable. If the machine is just cleaning up use-once cache, frequent events make no sense. And if the machine is struggling, the notifications better be in time. That's hardly a tunable. It's a factor that needs constant dynamic adjustment depending on VM state. The same state this mechanism is supposed to report. If we can't get this right, how will userspace? A better approach here would be to 1) find a minimum window size that makes us confident that there are no false positives - this is likely to be based on machine size, maybe the low watermark? - and 2) limit reporting of lower levels, so you're not flooded with ALLGOOD! events. VMPRESSURE_CRITICAL: report every vmpressure_win VMPRESSURE_MEDIUM: report every vmpressure_win*2 VMPRESSURE_LOW: report every vmpressure_win*4 Pick your favorite scaling factor here.