linux-kernel - Re: [RFC PATCH 1/2] Shrink compat

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Message-Id: <1222731482.23159.23.camel@calx>
Date:	Mon, 29 Sep 2008 18:38:02 -0500
From:	Matt Mackall <mpm@...enic.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Pavel Machek <pavel@...e.cz>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] Shrink compat_ioctl.c


On Tue, 2008-09-30 at 01:38 +0200, Andi Kleen wrote:
> On Mon, Sep 29, 2008 at 06:27:10PM -0500, Matt Mackall wrote:
> > I'm throwing this out untested as I don't have a mixed 64/32 system
> > handy at the moment.
> 
> Makes sense (assuming it works, haven't tested)
> 
> > compat_ioctl: shrink structures
> > 
> > Initially, compat_ioctl.c has a ~18k table of translated ioctls.
> > Each table entry is 24 bytes but we can shrink this to 16:
> > 
> > - use short table indexes rather than a pointer for .next values
> > - use unsigned ints for cmd numbers (they're 32-bit ioctls, after all)
> > 
> > In addition, there's a 2k hash table that we can do away with simply
> > by hashifying the main table in place at init time.
> 
> You mean by using a closed hash?

The original hash table was 256 pointers into the main table. I simply
rearrange the main table so the first 256 entries have an appropriate
hash. Basically:

for i in len(table):
  h = hash(table[i])
  swap(table[i], table[hash])

At the end of this loop, table[0:256] will contain an appropriate table
entry, if it exists. So no secondary table is needed.

-- 
Mathematics is the supreme nostalgia of our time.

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