RE: [PATCH] mm: moving dirty pages balancing to pdfludh entirely

["Ananiev, Leonid I" <leonid.i.ananiev@intel.com>]

Nikita Danilov wtites:
>> Pdflush thread functions as before patching. Pdflush tends to make
pages
>> un-dirty without overload memory or IO and it is not need to let
pdflush

> This assumption is valid for ext2

The assumption that pdflush should to make pages un-dirty without
overload memory or IO is not for ext2 but for it sense. I'm working with
ext3. A lot of work it does while writepages(). pdflush is throttled:
while vmscan have sorted 32 page for paging-out it calls
blk_congestion_wait() nevertheless had it put one of 32 page into
congested queue or had not. pdflush is throttled.

Leonid
 

-----Original Message-----
From: Nikita Danilov [mailto:nikita@clusterfs.com] 
Sent: Tuesday, July 04, 2006 1:55 PM
To: Ananiev, Leonid I
Cc: Linux Kernel Mailing List
Subject: Re: [PATCH] mm: moving dirty pages balancing to pdfludh
entirely

Ananiev, Leonid I writes:
 > Nikita Danilov wtites:
 > > performs page-out even if queue is congested.
 > 	Yes. If user thread which generates dirty pages need in
 > reclaimed memory it consider own dirty page as candidate for
page-out.
 > It functions as before patching.
 > 
 > > Intent of this is to throttle writers.
 > I suppose you means dirtier or write(2) caller but not writepage()
 > caller. The dirtier  is throttled  with backing_dev_info logic as
before
 > patching.

I meant ->writepages() used by balance_dirty_pages(), see below.

 > 
 > 	While pdflush thread sorts pages for page-out it does not
 > consider as a candidate a page to be written with congested queue.
 > Pdflush thread functions as before patching. Pdflush tends to make
pages
 > un-dirty without overload memory or IO and it is not need to let
pdflush

This assumption is valid for ext2, where ->writepages() simply sends
pages to the storage, but other file systems (like reiser4) do a *lot*
of work in ->writepages() path, allocating quite an amount of memory
before starting write-out. With your patch, this work is done from
pdflush, and won't be throttled by may_write_to_queue() check, thus
increasing a risk of allocation failure.

 > do page-out with congested queue as you have proposed.
 > 	
 > Leonid

Nikita.
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