Re: [PATCH V2 0/7] Cleancache (was Transcendent Memory): overview

[Minchan Kim <minchan.kim@gmail.com>]

Hi, Nitin. 

I am happy to hear you started this work. 

On Fri, Jun 04, 2010 at 03:06:49PM +0530, Nitin Gupta wrote:
> On 06/03/2010 09:13 PM, Dan Magenheimer wrote:
> >> On 06/03/2010 10:23 AM, Andreas Dilger wrote:
> >>> On 2010-06-02, at 20:46, Nitin Gupta wrote:
> >>
> >>> I was thinking it would be quite clever to do compression in, say,
> >>> 64kB or 128kB chunks in a mapping (to get decent compression) and
> >>> then write these compressed chunks directly from the page cache
> >>> to disk in btrfs and/or a revived compressed ext4.
> >>
> >> Batching of pages to get good compression ratio seems doable.
> > 
> > Is there evidence that batching a set of random individual 4K
> > pages will have a significantly better compression ratio than
> > compressing the pages separately?  I certainly understand that
> > if the pages are from the same file, compression is likely to
> > be better, but pages evicted from the page cache (which is
> > the source for all cleancache_puts) are likely to be quite a
> > bit more random than that, aren't they?
> > 
> 
> 
> Batching of pages from random files may not be so effective but
> it would be interesting to collect some data for this. Still,
> per-inode batching of pages seems doable and this should help
> us get over this problem.

1)
Please, consider system memory pressure case. 
In such case, we have to release compressed cache pages. 
Or it would be better to discard not-good-compression pages 
when you compress it. 

2)
This work is related to page reclaiming.
Page reclaiming is to make free memory. 
But this work might free memory little than old. 
I admit your concept is good in terms of I/O cost. 
But we might discard more clean pages than old if you want to 
do batching of pages for good compression.

3)
testcase. 

As I mentioned, it could be good in terms of I/O cost. 
But it could change system's behavior due to page consumption of backend. 
so many page scanning/reclaiming could be happen. 
It means hot pages can be discarded with this patch.
But it's a just guessing. 
So we need number with testcase we can measure I/O and system 
responsivness. 

> 
> Thanks,
> Nitin

-- 
Kind regards,
Minchan Kim

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