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.
Unused memory is wasted memory, hence the reason for FS caches.
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.
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.
Agreed, but yet again it comes back to not having enough memory for *performance*.let me put it a completely other way: swap outs are normal, fairlyPlease explain how swapping out a page is good for performance.
frequent, and 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.
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,
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