[PATCH] mm: ZAP_BLOCK causes redundant work
The address based work estimate for unmapping (for lockbreak) is and always
was horribly inefficient for sparse mappings. The problem is most simply
explained with an example:
If we find a pgd is clear, we still have to call into unmap_page_range
PGDIR_SIZE / ZAP_BLOCK_SIZE times, each time checking the clear pgd, in
order to progress the working address to the next pgd.
The fundamental way to solve the problem is to keep track of the end
address we've processed and pass it back to the higher layers.
From: Nick Piggin <npiggin@suse.de>
Modification to completely get away from address based work estimate
and instead use an abstract count, with a very small cost for empty
entries as opposed to present pages.
On 2.6.14-git2, ppc64, and CONFIG_PREEMPT=y, mapping and unmapping 1TB
of virtual address space takes 1.69s; with the following patch applied,
this operation can be done 1000 times in less than 0.01s
From: Andrew Morton <akpm@osdl.org>
With CONFIG_HUTETLB_PAGE=n:
mm/memory.c: In function `unmap_vmas':
mm/memory.c:779: warning: division by zero
Due to
zap_work -= (end - start) /
(HPAGE_SIZE / PAGE_SIZE);
So make the dummy HPAGE_SIZE non-zero
Signed-off-by: Robin Holt <holt@sgi.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
2 files changed
