Re: [PATCH] mm: consider all swapped back pages in used-once logic

[Zheng Liu <gnehzuil.liu@gmail.com>]

On Mon, May 21, 2012 at 11:37:05AM +0200, Johannes Weiner wrote:
[snip]
> > > Is it because the read()/write() IO is high throughput and pushes
> > > pages through the LRU lists faster than the mmap pages are referenced?
> > 
> > Yes, in this application, one query needs to access mapped file page
> > twice and file page cache twice.  Namely, one query needs to do 4 disk
> > I/Os.  We have used fadvise(2) to reduce file page cache accessing to
> > only once.  For mapped file page, in fact them are accessed only once
> > because in one query the same data is accessed twice.  Thus, one query
> > causes 2 disk I/Os now.  The size of read/write is quite larger than
> > mmap/munmap.  So, as you see, if we can keep mmap/munmap file in memory
> > as much as possible, we will gain the better performance.
> 
> You access the same unmapped cache twice, i.e. repeated reads or
> writes against the same file offset?

No.  We access the same mapped file twice.

> 
> How do you use fadvise?

We access the header and content of the file respectively using read/write.
The header and content are sequentially.  So we use fadivse(2) with
FADV_WILLNEED flag to do a readahead.

> > In addition, another factor also has some impacts for this application.
> > In inactive_file_is_low_global(), it is different between 2.6.18 and
> > upstream kernel.  IMHO, it causes that mapped file pages in active list
> > are moved into inactive list frequently.
> > 
> > Currently, we add a parameter in inactive_file_is_low_global() to adjust
> > this ratio.  Meanwhile we activate every mapped file pages for the first
> > time.  Then the performance gets better, but it still doesn't reach the
> > performance of 2.6.18.
> 
> 2.6.18 didn't have the active list protection at all and always
> forcibly deactivated pages during reclaim.  Have you tried fully
> reverting to this by making inactive_file_is_low_global() return true
> unconditionally?

No, I don't try it.  AFAIK, 2.6.18 didn't protect the active list.  But
it doesn't always forcibly deactivate the pages.  I remember that in
2.6.18 kernel we calculate 'mapped_ratio' in shrink_active_list(), and
then we get 'swap_tendency' according to 'mapped_ratio', 'distress', and
'sc->swappiness'.  If 'swap_tendency' is not greater than 100.  It
doesn't reclaim mapped file pages.  By this equation, if the sum of the
anonymous pages and mapped file pages is not greater than the 50% of
total pages, we don't deactivate these pages.  Am I missing something?

> Could it be that by accessing your "used-once" unmapped cache twice in
> short succession, you accidentally activate it all?

It could not happen.  Certainly it is possible to access a file twice at
the same offset in product system.  That is reason why we use buffered
IO rather than direct IO.  But in testing system we could not access the
same file twice at the same offset.

> Thereby having ONLY mapped file pages on the inactive list, adding to
> the pressure on them?
> 
> And, by having the wrong pages on the active list, actually benefit
> from the active list not being protected from inactive list cycle
> speed and instead pushed out quickly again?

Sorry, you mean that in 2.6.18 kernel it benefits from the wrong pages
on the active list, isn't it?

Regards,
Zheng

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