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Date: Mon, 07 May 2007 10:18:35 -0400 From: Bill Davidsen <davidsen@....com> To: Ingo Molnar <mingo@...e.hu> CC: Nick Piggin <nickpiggin@...oo.com.au>, Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, Con Kolivas <kernel@...ivas.org> Subject: Re: swap-prefetch: 2.6.22 -mm merge plans Ingo Molnar wrote: > * Nick Piggin <nickpiggin@...oo.com.au> 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 <davidsen@....com> "We have more to fear from the bungling of the incompetent than from the machinations of the wicked." - from Slashdot - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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