Re: [PATCH RFC] mm/readahead: improve randread performance with readahead disabled

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi,

在 2025/07/14 9:42, Yu Kuai 写道:
> Hi,
> 
> 在 2025/07/01 19:08, Yu Kuai 写道:
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> We have a workload of random 4k-128k read on a HDD, from iostat we 
>> observed
>> that average request size is 256k+ and bandwidth is 100MB+, this is 
>> because
>> readahead waste lots of disk bandwidth. Hence we disable readahead and
>> performance from user side is indeed much better(2x+), however, from
>> iostat we observed request size is just 4k and bandwidth is just around
>> 40MB.
>>
>> Then we do a simple dd test and found out if readahead is disabled,
>> page_cache_sync_ra() will force to read one page at a time, and this
>> really doesn't make sense because we can just issue user requested size
>> request to disk.
>>
>> Fix this problem by removing the limit to read one page at a time from
>> page_cache_sync_ra(), this way the random read workload can get better
>> performance with readahead disabled.
>>
>> PS: I'm not sure if I miss anything, so this version is RFC
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
>> ---
>>   mm/readahead.c | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
>>
> 
> Friendly ping ...
> 

Friendly ping again ...

>> diff --git a/mm/readahead.c b/mm/readahead.c
>> index 20d36d6b055e..1df85ccba575 100644
>> --- a/mm/readahead.c
>> +++ b/mm/readahead.c
>> @@ -561,13 +561,21 @@ void page_cache_sync_ra(struct readahead_control 
>> *ractl,
>>        * Even if readahead is disabled, issue this request as readahead
>>        * as we'll need it to satisfy the requested range. The forced
>>        * readahead will do the right thing and limit the read to just the
>> -     * requested range, which we'll set to 1 page for this case.
>> +     * requested range.
>>        */
>> -    if (!ra->ra_pages || blk_cgroup_congested()) {
>> +    if (blk_cgroup_congested()) {
>>           if (!ractl->file)
>>               return;
>> +        /*
>> +         * If the cgroup is congested, ensure to do at least 1 page of
>> +         * readahead to make progress on the read.
>> +         */
>>           req_count = 1;
>>           do_forced_ra = true;
>> +    } else if (!ra->ra_pages) {
>> +        if (!ractl->file)
>> +            return;
>> +        do_forced_ra = true;
>>       }
>>       /* be dumb */
>>
> 
> .
>