Re: [PATCH] ext4: group cache is added in ext4_mb_discard_preallocations()

[jing zhang <zj.barak@gmail.com>]

2010/3/31, Aneesh Kumar K. V <aneesh.kumar@linux.vnet.ibm.com>:
> On Tue, 30 Mar 2010 20:36:17 +0800, jing zhang <zj.barak@gmail.com> wrote:
>> From: Jing Zhang <zj.barak@gmail.com>
>>
>> Date: Tue Mar 30 20:35:22     2010
>>
>> With the added cache, better group locality may be earned when
>> allocating blocks.
>>
>> Cc: Theodore Ts'o <tytso@mit.edu>
>> Cc: Andreas Dilger <adilger@sun.com>
>> Cc: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
>> Cc: "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com>
>> Signed-off-by: Jing Zhang <zj.barak@gmail.com>
>>
>> ---
>>
>> --- linux-2.6.32/fs/ext4/mballoc.c	2009-12-03 11:51:22.000000000 +0800
>> +++ ext4_mm_leak/mballoc-13.c	2010-03-30 20:28:08.000000000 +0800
>> @@ -4183,12 +4183,20 @@ static int ext4_mb_discard_preallocation
>>  	ext4_group_t i, ngroups = ext4_get_groups_count(sb);
>>  	int ret;
>>  	int freed = 0;
>> +	static ext4_group_t grp_cache = 0;
>>
>>  	trace_ext4_mb_discard_preallocations(sb, needed);
>> -	for (i = 0; i < ngroups && needed > 0; i++) {
>> -		ret = ext4_mb_discard_group_preallocations(sb, i, needed);
>> +	if (needed <= 0)
>> +		return freed;
>> +	for (i = 0; i < ngroups; i++) {
>> +		if (grp_cache >= ngroups)
>> +			grp_cache -= ngroups;
>> +		ret = ext4_mb_discard_group_preallocations(sb, grp_cache, needed);
>>  		freed += ret;
>>  		needed -= ret;
>> +		if (needed <= 0)
>> +			break;
>> +		grp_cache++;
>>  	}
>>
>>  	return freed;
>
> can you explain this further ?
>
> -aneesh
>

The added cache checks whether blocks pre-allocated in group are still
available. If yes, they are discarded and used for allocation without
change of group. So more group locality can be earned.

What is more, in function, ext4_mb_discard_group_preallocations(),
pre-allocation is allowed  to be discarded as much as possible by
yielding.

     - zj