From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nathan Dietsch Subject: Re: RAID5 crash and burn Date: Mon, 01 Nov 2004 19:58:01 +1100 Message-ID: <4185FA99.9070101@ndietsch.com> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Mark Hahn , linux-raid@vger.kernel.org List-Id: linux-raid.ids Mark, Mark Hahn wrote: >> From Understanding the Linux Kernel "Every access by a program to a >>page that is swapped out increases the process execution time by an >>order of magnitude. In short, if performance is of great importance, >>swapping should be considered as a last resort." (p. 529) >> >> > >not relevant: we're talking about what it costs to perform >a swapout, not what happens if the wrong page is chosen for >swapout, and later necessitates a swapin. > > You have convinced me that swapping out is low-overhead in Linux. > > >>I agree that the write is cheap compared to the read (where you will >>stall), but what is the purpose of writing something to swap if you do >>not plan to read it again? >> >> > >to free the underlying physical page for other use. > > Unused memory is wasted memory, hence the reason for FS caches. If you have such a squeeze on memory that you are swapping for a more active page using LRU algorithms, you do not have enough memory for the task at hand. >>>let me put it a completely other way: swap outs are normal, fairly >>>frequent, and GOOD for performance. >>> >>> >>> >>Please explain how swapping out a page is good for performance. >> >> > >swapouts mean that the kernel doesn't need to dedicate a page of >physical memory to keep track of some stale page. swapping out >active pages would be mistake, obviously. the kernel tries to >avoid that mistake in any OS. > > Agreed, but yet again it comes back to not having enough memory for *performance*. > > >>I understand that unused pages are paged out, but how does this improve >>performance beyond making more room for the FS cache? >> >> > >a swapout frees a physical page, period. whether it's used for >the FS cache or not is irrelevant. perhaps your model for how >the system works involves uniform references to all pages? >if that were true, then indeed, swapouts would be dubious. >since references are highly skewed, there are some pages which >are not effective investments of physical ram, so to speak, >but which can't be thrown away (because they're dirty and/or anonymous). > > > >>I am simply saying that comparing RAID5 and RAID1 for the purposes of >>swapping is irrelevant, because if you are relying on the performance of >>your swap device, you are missing the point that you have run out of >>memory. >> >> > >which is either vapid or wrong. yes, it's silly to worry about >how fast your system thrashes. but no one is talking about non-fatal error >conditions like thrashing. and there *is* a good reason to choose >r1 over r5 for swap, since writes are so much lower-overhead. worrying >about the efficiency of r1 vs r5 reads verges on the "optimizing >for thrashing" that you're so worried about. > > That is what I was getting at. I think this is going in circles, but it has made me see swapping in another light. Regards, Nathan