potential for buffer_head shrinkage.

potential for buffer_head shrinkage.

[Dave Jones]

Looking at struct buffer_head, it seems that b_state
uses at most 15 bits, where it's defined as a 64bit entity
due to it being used by bit_spin_lock and friends.

Given it's not uncommon for a few hundred thousand of these
to be present, I wonder if it's worth the effort of folding
b_count into the upper bits of b_state, thus shrinking
buffer_head by 16 bits?  This would still leave 32 bits
'wasted' for further bh_state_bits expansion if necessary.

Opinions?

		Dave

-- 
http://www.codemonkey.org.uk

Re: potential for buffer_head shrinkage.

[Dave Jones]

On Fri, Dec 29, 2006 at 09:45:54PM -0500, Dave Jones wrote:
 > Looking at struct buffer_head, it seems that b_state
 > uses at most 15 bits, where it's defined as a 64bit entity
 > due to it being used by bit_spin_lock and friends.
 > 
 > Given it's not uncommon for a few hundred thousand of these
 > to be present, I wonder if it's worth the effort of folding
 > b_count into the upper bits of b_state, thus shrinking
 > buffer_head by 16 bits?  This would still leave 32 bits
 > 'wasted' for further bh_state_bits expansion if necessary.

My math here based on a 64 bit compile btw in case that wasn't obvious.
32 bit wouldn't leave room for expansion.

		Dave

-- 
http://www.codemonkey.org.uk