Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Tue, 8 Jan 2002 18:06:48 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Tue, 8 Jan 2002 18:06:38 -0500 Received: from Expansa.sns.it ([192.167.206.189]:15114 "EHLO Expansa.sns.it") by vger.kernel.org with ESMTP id ; Tue, 8 Jan 2002 18:06:20 -0500 Date: Wed, 9 Jan 2002 00:02:48 +0100 (CET) From: Luigi Genoni To: Daniel Phillips cc: Andrea Arcangeli , Anton Blanchard , Dieter N?tzel , Marcelo Tosatti , Rik van Riel , Linux Kernel List , Andrew Morton , Robert Love Subject: Re: [2.4.17/18pre] VM and swap - it's really unusable In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 8 Jan 2002, Daniel Phillips wrote: > On January 8, 2002 04:29 pm, Andrea Arcangeli wrote: > > > The preemptible approach is much less of a maintainance headache, since > > > people don't have to be constantly doing audits to see if something changed, > > > and going in to fiddle with scheduling points. > > > > this yes, it requires less maintainance, but still you should keep in > > mind the details about the spinlocks, things like the checks the VM does > > in shrink_cache are needed also with preemptive kernel. > > Yes of course, the spinlock regions still have to be analyzed and both > patches have to be maintained for that. Long duration spinlocks are bad > by any measure, and have to be dealt with anyway. > > > > Finally, with preemption, rescheduling can be forced with essentially zero > > > latency in response to an arbitrary interrupt such as IO completion, whereas > > > the non-preemptive kernel will have to 'coast to a stop'. In other words, > > > the non-preemptive kernel will have little lags between successive IOs, > > > whereas the preemptive kernel can submit the next IO immediately. So there > > > are bound to be loads where the preemptive kernel turns in better latency > > > *and throughput* than the scheduling point hack. > > > > The I/O pipeline is big enough that a few msec before or later in a > > submit_bh shouldn't make a difference, the batch logic in the > > ll_rw_block layer also try to reduce the reschedule, and last but not > > the least if the task is I/O bound preemptive kernel or not won't make > > any difference in the submit_bh latency because no task is eating cpu > > and latency will be the one of pure schedule call. > > That's not correct. For one thing, you don't know that no task is eating > CPU, or that nobody is hogging the kernel. Look at the above, and consider > the part about the little lags between IOs. > > > > Mind you, I'm not devaluing Andrew's work, it's good and valuable. However > > > it's good to be aware of why that approach can't equal the latency-busting > > > performance of the preemptive approach. > > > > I also don't want to devaluate the preemptive kernel approch (the mean > > latency it can reach is lower than the one of the lowlat kernel, however > > I personally care only about worst case latency and this is why I don't > > feel the need of -preempt), > > This is exactly the case that -preempt handles well. On the other hand, > trying to show that scheduling hacks satisfy any given latency bound is > equivalent to solving the halting problem. > > I thought you had done some real time work? > > > but I just wanted to make clear that the > > idea that is floating around that preemptive kernel is all goodness is > > very far from reality, you get very low mean latency but at a price. > > A price lots of people are willing to pay Probably sometimes they are not making a good business. In the reality preempt is good in many scenarios, as I said, and I agree that for desktops, and dedicated servers where just one application runs, and probably the CPU is idle the most of the time, indeed users have a speed feeling. Please consider that on eavilly loaded servers, with 40 and more users, some are running gcc, others g77, others g++ compilations, someone runs pine or mutt or kmail, and netscape, and mozilla, and emacs (someone form xterm kde or gnome), and and and... You can have also 4/8 CPU butthey are not infinite ;) (but I talk mainly thinking of dualAthlon systems). there is a lot of memory and disk I/O. This is not a strange scenary on the interactive servers used at SNS. Here preempt has a too high price > > By the way, have you measured the cost of -preempt in practice? > Yes, I did a lot of tests, and with current preempt patch definitelly I was seeing a too big performance loss. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/