Subject: Re: [RFC] [-mm] Remove 'unsafe' LZO decompressor
From: Richard Purdie <richard@openedhand.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Michael-Luke Jones <mlj28@cam.ac.uk>, lkml <linux-kernel@vger.kernel.org>,
       Satyam Sharma <satyam.sharma@gmail.com>,
       Nitin Gupta <nitingupta910@gmail.com>,
       "Markus F.X.J. Oberhumer" <markus@oberhumer.com>
In-Reply-To: <20070524115055.6be93924.akpm@linux-foundation.org>
References: <1412F214-FA89-4088-B3A2-DEE03C324FB6@cam.ac.uk>
	 <20070524115055.6be93924.akpm@linux-foundation.org>
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Date: Thu, 24 May 2007 23:26:12 +0100
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On Thu, 2007-05-24 at 11:50 -0700, Andrew Morton wrote:
> On Thu, 24 May 2007 18:15:17 +0100
> Michael-Luke Jones <mlj28@cam.ac.uk> wrote:
> 		
> > Attached is a patch which may be desirable for -mm. It applies  
> > directly to 2.6.22-rc2-mm1.
> > 
> > The patch removes the 'unsafe' LZO decompression function, lowering  
> > the size of the minilzo.c file by nearly 500 out of an original 1727  
> > lines. It also removes references to the 'unsafe' decompression  
> > function in the public LZO header and the EXPORT_SYMBOL_GPL declaration.
[...]
> > Comments / disagreement all welcome :)
> 
> This is obviously a highly desirable thing to do for a number of reasons. 
> But have we quantified the performance difference?

Ok, I've done some tests: 

1. Comparing the safe and unsafe functions

For my minilzo kernel patch, the safe version showed a 7.2% performance
hit. For Nitin's patch, it showed a 3.2% performance hit (but see
below).

Could be a lot worse and I don't object to the removal of the unsafe
version.

2. Comparing Nitin's code with my minilzo based kernel patch.

My kernel patch is about 2.25 times faster at decompression (16725Kb/ms
vs 7399Kb/ms) and fractionally faster at compression (1434kb/ms vs
1490kb/ms). As things stand the performance of Nitin's patch is
therefore unacceptable as that is a significant decompression
performance loss.

These tests are on 32 bit Intel and in userspace but I've found them to
be a pretty good indicator of what happens in the real world and on
other architectures. 
For simplicity I made these tests with some existing code I had around
but its licence is such I can't share it, much to my frustration.

Cheers,

Richard

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