linux-ext4 - Re: [PATCH 1/2] ext4: mballoc

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Date:   Sat, 30 May 2020 05:01:40 +0000
From:   Alex Zhuravlev <azhuravlev@...mcloud.com>
To:     Благодаренко Артём 
        <artem.blagodarenko@...il.com>
CC:     "linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 1/2] ext4:  mballoc - prefetching for bitmaps


Hi

> On 29 May 2020, at 19:19, Благодаренко Артём <artem.blagodarenko@...il.com> wrote:
> 
> Also, we have encountered directory creating rate drop with this (not exact this, but Lustre FS version) patch. From 70-80K to 30-40K.
> Excluding this patch restore rates to the original values.
> I am investigating it now. Alex, do you expect this optimisation has impact to names creation?
> Is plenty of files and directories creation corner case for this optimisation?

Noticed as well, the last version posted to the list should have this problem fixed.

> 
> Can be useful giving an ability to disable this optimisation? As option, by setting s_mb_prefetch to zero.
> Now 0 at s_mb_prefetch allows to skip the optimisation for cr=0 and cr=1. 

s_mb_prefetch_limit=0 can be used to disable prefetching?

>> +	/* limit prefetching at cr=0, otherwise mballoc can
>> +	 * spend a lot of time loading imperfect groups */
>> +	if (ac->ac_criteria < 2 && ac->ac_prefetch_ios >= sbi->s_mb_prefetch_limit)
>> +		return;
> 
> A comment above says prefetching is limited for cr=0 but code limit it for cr=0 and cr=1.
> Do you need change the comment or code?

OK

>> +	if (sbi->s_mb_prefetch > ext4_get_groups_count(sb))
>> +		sbi->s_mb_prefetch = ext4_get_groups_count(sb);
>> +	/* now many real IOs to prefetch within a single allocation at cr=0
> 
> Do you mean “how many” here? At cr=0 and cr=1?

Yes, of course


Thanks, Alex