(12/29/11 6:27 PM), Hugh Dickins wrote:
On Thu, 29 Dec 2011, Andrew Morton wrote:
On Wed, 28 Dec 2011 20:39:36 -0800 (PST)
Hugh Dickins<hughd@xxxxxxxxxx> wrote:
Replace pagevecs in putback_lru_pages() and move_active_pages_to_lru()
by lists of pages_to_free
One effect of the pagevec handling was to limit lru_lock hold times and
interrupt-disabled times.
This patch removes that upper bound and has the potential to cause
various latency problems when processing large numbers of pages.
The affected functions have rather a lot of callers. I don't think
that auditing all these callers and convincing ourselves that none of
them pass in 10,000 pages is sufficient, because that doesn't prevent us
from introducing such latency problems as the MM code evolves.
That's an interesting slant on it, that hadn't crossed my mind;
but it looks like intervening changes have answered that concern.
putback_lru_pages() has one caller, shrink_inactive_list();
move_active_pages_to_lru() has one caller, shrink_active_list().
Following those back, they're in all cases capped to SWAP_CLUSTER_MAX
pages per call. That's 32 pages, not so very much more than the 14
page limit the pagevecs were imposing.
And both shrink_inactive_list() and shrink_active_list() gather these
pages with isolate_lru_pages(), which does not drop lock or enable
interrupts at all - probably why the SWAP_CLUSTER_MAX cap got imposed.
When lumpy reclaim occur, isolate_lru_pages() gather much pages than
SWAP_CLUSTER_MAX. However, at that time, I think this patch behave
better than old. If we release and retake zone lock per 14 pages,
other tasks can easily steal a part of lumpy reclaimed pages. and then
long latency wrongness will be happen when system is under large page
memory allocation pressure. That's the reason why I posted very similar
patch a long time ago.
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