Messages in this thread | | | Date | Sat, 18 Aug 2001 16:15:21 +0100 (BST) | From | Mark Hemment <> | Subject | kswap spinning |
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Hi,
Jumping from 2.4.7 to 2.4.9 has shown up a problem with the VM balancing code. Under load, I've seen kswapd become a CPU hog (on a 5GB box). Now that I've got a theory, I cannot reproduce the problem for confirmation, but here is the theory anyway.
The tests in free_shortage() and inactive_shortage() assume that the memory state for a zone can be improved by calling refill_inactive_scan(), page_launder(), the inode/dcache shrinking functions, and the general slab-cache reaping func. Usually, some combination of the reaping functions will improve a zone's state - but not always.
The problem is the DMA zone. As it is (relatively) small on some archs (IA-32 for example), it is possible that almost all pages from this zone are being used by a sub-systems which simply won't give them up. Examples of use are; vmalloc()ed page for modules, pre-allocated socket buffs for NICs, task-structs/kernel-stack - you get the idea.
In the case I believe I'm seeing, both free_shortage() and inactive_shortage() are returning a shortage for the DMA zone which triggers all the work in do_try_to_free_pages(). But as there aren't any DMA pages left in the page-cache, being used as anonymous pages, or being used in the other places the code looks, do_try_to_free_pages() returns non-zero to kswapd() and off we go again. This also causes callers of try_to_free_pages() (from __alloc_pages()) to suck CPU cycles as well.
On HIGHMEM boxes, it is possible for the NORMAL zone to get into the same state (although v unlikely).
I'd rather not get into having specialist code in mm/vmscan.c (or arch specific code) to handle a small DMA zone - so what are the other solutions? (Assuming the above theory holds true.)
One possible solution is not to give DMA memory out, except for; o explicit DMA allocations o page-cache, anonymous page, allocations assuming explicit DMA allocations are low, we'll at least know the remaining DMA pages are somewhere we can get at them - would need to be trigger by an arch specific flag for those that don't have a "tiny" zone. However, I don't like this, feels like fixing the wrong problem. :(
Mark
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