Re: kernel 2.6.13 buffer strangeness - FIXED

[Anthony Wesley <awesley@acquerra.com.au>]

Nate Diller wrote:

> just found the culprit.  guess i should have read the code the first 
> time.  get_dirty_limits() in drivers/block/page_writeback.c has a 
> hard-coded upper limit to dirty_ratio.  it's capped to half of the 
> unmapped pages, so maybe 30-40% of your system's memory.  so if you are 
> brave, just remove the "/ 2" parts from the 'if (dirty_ratio > 
> unmapped_ratio / 2) dirty_ratio = unmapped_ratio / 2;' check, and you 
> can have all the OOM goodness you want.

Well well well.

Thank you Nate!

I changed that bit of code to:

	if (dirty_ratio > unmapped_ratio - 10) 
		dirty_ratio = unmapped_ratio - 10;

and added a couple of sanity checks so that it couldn't get below 5 or above 95.

Then set /proc/sys/vm/dirty_ratio to 95 and dirty_background_ratio to 1.

Guess what? Now my video streams for 120 seconds before being throttled. This is *much better*.
It's now quite feasible to reach 30 seconds by finding a faster disk and/or adding another 512M
or RAM.

I guess the hard-coded limit is in there for a reason, but I sure wish it was adjustable without
this sort of hacking.

regards, Anthony

-- 
Anthony Wesley
Director and IT/Network Consultant
Smart Networks Pty Ltd
Acquerra Pty Ltd

Anthony.Wesley@acquerra.com.au
Phone: (02) 62595404 or 0419409836