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

[Yu Kuai <yukuai1@huaweicloud.com>]

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 ...

> 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 */
>