Re: [PATCH] count for buffer IO in page_launder()

[Marcelo Tosatti <marcelo@conectiva.com.br>]

On Fri, 2 Mar 2001, Stephen C. Tweedie wrote:

> Hi,
> 
> On Tue, Feb 27, 2001 at 04:09:09AM -0300, Marcelo Tosatti wrote:
> > 
> > page_launder() is not counting direct ll_rw_block() IO correctly in the
> > flushed pages counter. 
> 
> Having not seen any follow to this, it's worth asking: what is the
> expected consequence of _not_ including this?  

The page launder loop avoids flushing too many pages if it already
flushed/cleaned enough pages to remove the system from low memory
condition (mm/vmscan.c::page_launder()):

                /*
                 * Disk IO is really expensive, so we make sure we
                 * don't do more work than needed.
                 * Note that clean pages from zones with enough free
                 * pages still get recycled and dirty pages from these
                 * zones can get flushed due to IO clustering.
                 */
                if (freed_pages + flushed_pages > target && !free_shortage())
                        break;


Dirty buffer pages and dirty pagecache pages with page->buffers mapping
which were being flushed (with try_to_free_buffers()) were not being
counted in the "flushed_pages" counter correctly.

So what could happen is that tasks trying to launder pages could
flush/swapout more than needed. 

> Have you done an performance testing on it?

No. The code makes sense now.

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