linux-ext4 - Re: [PATCH] ext4: preload block group descriptors

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Date:   Tue, 28 Feb 2017 19:29:53 +0300
From:   Alexey Lyashkov <alexey.lyashkov@...il.com>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Artem Blagodarenko <artem.blagodarenko@...il.com>,
        linux-ext4@...r.kernel.org,
        Andrew Perepechko <andrew.perepechko@...gate.com>
Subject: Re: [PATCH] ext4: preload block group descriptors


> 28 февр. 2017 г., в 19:19, Darrick J. Wong <darrick.wong@...cle.com> написал(а):
> 
> On Mon, Feb 20, 2017 at 07:45:53PM +0300, Artem Blagodarenko wrote:
>> From: Andrew Perepechko <andrew.perepechko@...gate.com>
>> 
>> With enabled meta_bg option block group descriptors
>> reading IO is not sequential and requires optimization.
>> 
>> Signed-off-by: Andrew Perepechko <andrew.perepechko@...gate.com>
>> ---
>> fs/ext4/super.c | 6 ++++++
>> 1 file changed, 6 insertions(+)
>> 
>> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
>> index a673558..0e52c91 100644
>> --- a/fs/ext4/super.c
>> +++ b/fs/ext4/super.c
>> @@ -3879,6 +3879,12 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
>> 
>> 	bgl_lock_init(sbi->s_blockgroup_lock);
>> 
>> +	/* Pre-read the descriptors into the buffer cache */
>> +	for (i = 0; i < db_count; i++) {
>> +		block = descriptor_loc(sb, logical_sb_block, i);
>> +		sb_breadahead(sb, block);
>> +	}
> 
> Why does the libext2fs patch start readahead only on the meta bgs
> whereas this patch starts it for all of them?  I suppose it's just as
> well that the kernel prefetches all of them, but I was curious. :)
> 
Just because a simplify code. 
libext2fs makes read a first gd porting via single read call, it fast.
while just meta_bg generate a random io load.
while a kernel part just a copy/paste a second loop :) Artem have a modified version for this patch,
when sb_bread_unmovable moved into ext4_check_descriptors to have chance increase window between read-ahead call and access to the buffer.
What about such change?


> --D
> 
>> +
>> 	for (i = 0; i < db_count; i++) {
>> 		block = descriptor_loc(sb, logical_sb_block, i);
>> 		sbi->s_group_desc[i] = sb_bread_unmovable(sb, block);
>> -- 
>> 1.8.3.1