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Date:   Sat, 30 May 2020 05:01:40 +0000
From:   Alex Zhuravlev <>
To:     Благодаренко Артём 
CC:     "" <>
Subject: Re: [PATCH 1/2] ext4:  mballoc - prefetching for bitmaps


> On 29 May 2020, at 19:19, Благодаренко Артём <> 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?


>> +	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

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