From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <463F353B.1040405@tmr.com> Date: Mon, 07 May 2007 10:18:35 -0400 From: Bill Davidsen MIME-Version: 1.0 Subject: Re: swap-prefetch: 2.6.22 -mm merge plans References: <20070430162007.ad46e153.akpm@linux-foundation.org> <20070503155407.GA7536@elte.hu> <463AE1EB.1020909@yahoo.com.au> <20070504085201.GA24666@elte.hu> In-Reply-To: <20070504085201.GA24666@elte.hu> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Ingo Molnar Cc: Nick Piggin , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Con Kolivas List-ID: Ingo Molnar wrote: > * Nick Piggin wrote: > >>> i'm wondering about swap-prefetch: > >> Being able to config all these core heuristics changes is really not >> that much of a positive. The fact that we might _need_ to config >> something out, and double the configuration range isn't too pleasing. > > Well, to the desktop user this is a speculative performance feature that > he is willing to potentially waste CPU and IO capacity, in expectation > of better performance. > > On the conceptual level it is _precisely the same thing as regular file > readahead_. (with the difference that to me swapahead seems to be quite > a bit more intelligent than our current file readahead logic.) > > This feature has no API or ABI impact at all, it's a pure performance > feature. (besides the trivial sysctl to turn it runtime on/off). > >> Here were some of my concerns, and where our discussion got up to. [...snip...] > i see no real problem here. We've had heuristics for a _long_ time in > various areas of the code. Sometimes they work, sometimes they suck. > > the flow of this is really easy: distro looking for a feature edge turns > it on and announces it, if the feature does not work out for users then > user turns it off and complains to distro, if enough users complain then > distro turns it off for next release, upstream forgets about this > performance feature and eventually removes it once someone notices that > it wouldnt even compile in the past 2 main releases. I see no problem > here, we did that in the past too with performance features. The > networking stack has literally dozens of such small tunable things which > get experimented with, and whose defaults do get tuned carefully. Some > of the knobs help bandwidth, some help latency. > I haven't looked at this code since it first came out and didn't impress me, but I think it would be good to get the current version in. However, when you say "user turns it off" I hope you mean "in /proc/sys with a switch or knob" and not by expecting people to recompile and install a kernel. Then it might take a little memory but wouldn't do something undesirable. Note: I had no bad effect from the code, it just didn't feel faster. On a low memory machine it might help. Of course I have wanted to have a hard limit on memory used for i/o buffers, just to avoid swapping programs to make room for i/o, so to some extent I feel as if this is a fix for a problem we shouldn't have. -- Bill Davidsen "We have more to fear from the bungling of the incompetent than from the machinations of the wicked." - from Slashdot -- 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