On Mon, Jul 23, 2007 at 07:00:59PM +1000, Nick Piggin wrote:
> Rusty Russell wrote:
> >On Sun, 2007-07-22 at 16:10 +0800, Fengguang Wu wrote:
>
> >>So I opt for it being made tunable, safe, and turned off by default.
>
> I hate tunables :) Unless we have workload A that gets a reasonable
> benefit from something and workload B that gets a significant regression,
> and no clear way to reconcile them...
Me too ;)
But sometimes we really want to avoid flushing the cache.
Andrew's user space LD_PRELOAD+fadvise based tool fit nicely here.
> >I'd like to see it turned on by default in -mm, and try to come up with
> >some server-like workload to measure the effect. Should be easy to
> >simulate something (eg. apache server, where clients grab some files in
> >preference, and apache server where clients grab different files).
>
> I don't like this kind of conditional information going from something
> like readahead into page reclaim. Unless it is for readahead _specific_
> data such as "I got these all wrong, so you can reclaim them" (which
> this isn't).
>
> Possibly it makes sense to realise that the given pages are cheaper
> to read back in as they are apparently being read-ahead very nicely.
In fact I have talked to Jens about it in last year's kernel summit.
The patch below explains itself.
---
Subject: cost based page reclaim
Cost based page reclaim - a minimalist implementation.
Suppose we cached 32 small files each with 1 page, and one 32-page chunk from a
large file. Should we first drop the 32-pages which are read in one I/O, or
drop the 32 distinct pages, each costs one I/O? (Given that the files are of
equal hotness.)
Page replacement algorithms should be designed to minimize the number of I/Os,
instead of the number of page faults. Dividing the cost of I/O by the number of
pages it bring in, we get the cost of the page. The bigger page cost, the more
'lives/bloods' the page should have.
This patch adds life to costly pages by pretending that they are
referenced more times. Possible downsides:
- burdens the pressure of vmscan
- active pages are no longer that 'active'
Signed-off-by: Fengguang Wu <[EMAIL PROTECTED]>
---
include/linux/backing-dev.h | 1 +
mm/readahead.c | 27 +++++++++++++++++++++++++++
mm/swap.c | 5 ++++-
3 files changed, 32 insertions(+), 1 deletion(-)
--- linux-2.6.22.orig/mm/readahead.c
+++ linux-2.6.22/mm/readahead.c
@@ -125,6 +125,7 @@ static void ra_account(struct file_ra_st
struct backing_dev_info default_backing_dev_info = {
.ra_pages = MAX_RA_PAGES,
+ .avg_ra_size = MAX_RA_PAGES * PAGE_CACHE_SIZE,
.ra_pages0 = INITIAL_RA_PAGES,
.ra_thrash_bytes = MAX_RA_PAGES * PAGE_CACHE_SIZE,
.state = 0,
@@ -133,6 +134,26 @@ struct backing_dev_info default_backing_
};
EXPORT_SYMBOL_GPL(default_backing_dev_info);
+#define log2(n) fls(n)
+static int readahead_cost_per_page(struct address_space *mapping,
+ unsigned long pages)
+{
+ unsigned long avg;
+ int cost;
+
+ avg = mapping->backing_dev_info->avg_ra_size;
+ avg = (pages * PAGE_CACHE_SIZE + avg * 1023) / 1024;
+ mapping->backing_dev_info->avg_ra_size = avg;
+
+ avg = avg / PAGE_CACHE_SIZE;
+ if (!avg || !pages)
+ cost = 0;
+ else
+ cost = (log2(avg) - log2(pages)) * 5 / log2(avg);
+
+ return cost;
+}
+
/*
* Initialise a struct file's readahead state. Assumes that the caller has
* memset *ra to zero.
@@ -418,6 +439,7 @@ __do_page_cache_readahead(struct address
struct page *page;
unsigned long end_index; /* The last page we want to read */
LIST_HEAD(page_pool);
+ int cost;
int page_idx;
int ret = 0;
loff_t isize = i_size_read(inode);
@@ -426,6 +448,7 @@ __do_page_cache_readahead(struct address
goto out;
end_index = ((isize - 1) >> PAGE_CACHE_SHIFT);
+ cost = readahead_cost_per_page(mapping, nr_to_read);
/*
* Preallocate as many pages as we will need.
@@ -448,6 +471,10 @@ __do_page_cache_readahead(struct address
if (!page)
break;
page->index = page_offset;
+ if (cost >= 3)
+ SetPageActive(page);
+ else if (cost == 2)
+ SetPageReferenced(page);
list_add(&page->lru, &page_pool);
if (page_idx == nr_to_read - lookahead_size)
SetPageReadahead(page);
--- linux-2.6.22.orig/mm/swap.c
+++ linux-2.6.22/mm/swap.c
@@ -365,7 +365,10 @@ void __pagevec_lru_add(struct pagevec *p
}
VM_BUG_ON(PageLRU(page));
SetPageLRU(page);
- add_page_to_inactive_list(zone, page);
+ if (!PageActive(page))
+ add_page_to_inactive_list(zone, page);
+ else
+ add_page_to_active_list(zone, page);
}
if (zone)
spin_unlock_irq(&zone->lru_lock);
--- linux-2.6.22.orig/include/linux/backing-dev.h
+++ linux-2.6.22/include/linux/backing-dev.h
@@ -26,6 +26,7 @@ typedef int (congested_fn)(void *, int);
struct backing_dev_info {
unsigned long ra_pages; /* max readahead in PAGE_CACHE_SIZE units */
+ unsigned long avg_ra_size;
unsigned long ra_pages0; /* min readahead on start of file */
unsigned long ra_thrash_bytes; /* estimated thrashing threshold */
unsigned long state; /* Always use atomic bitops on this */
-
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