> 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. > 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. > >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. > 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. > [0] Some longer running tasks (like batch jobs), that do not have > performance limitations can afford to swap. interesting way to think of it, since you're assuming that systems will always have as much memory as they can possibly use. all real systems have "memory pressure", which leads the VM to attempt top optimize which needs consume the limited, inadequate number of physical pages. swapouts are nothing more than the kernel trying to find a better (hotter) use for a page. pure capacity swapouts aren't really interesting, since there's really nothing you can do about them. - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
