Re: [dm-devel] [PATCH] dm-verity: Remove WQ_UNBOUND.

[Mikulas Patocka]

On Wed, 1 Feb 2023, Nathan Huckleberry wrote:

> Setting WQ_UNBOUND increases scheduler latency on ARM64.  This is likely
> due to the asymmetric architecture of ARM64 processors.
> 
> I've been unable to reproduce the results that claim WQ_UNBOUND gives a
> performance boost on x86-64.
> 
> This flag is causing performance issues for multiple subsystems within
> Android.  Notably, the same slowdown exists for decompression with
> EROFS.
> 
> | open-prebuilt-camera  | WQ_UNBOUND | ~WQ_UNBOUND   |
> |-----------------------|------------|---------------|
> | verity wait time (us) | 11746      | 119 (-98%)    |
> | erofs wait time (us)  | 357805     | 174205 (-51%) |
> 
> | sha256 ramdisk random read | WQ_UNBOUND    | ~WQ_UNBOUND |
> |----------------------------|-----------=---|-------------|
> | arm64 (accelerated)        | bw=42.4MiB/s  | bw=212MiB/s |
> | arm64 (generic)            | bw=16.5MiB/s  | bw=48MiB/s  |
> | x86_64 (generic)           | bw=233MiB/s   | bw=230MiB/s |
> 
> Cc: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
> Signed-off-by: Nathan Huckleberry <nhuck@xxxxxxxxxx>
> ---
>  drivers/md/dm-verity-target.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> index ccf5b852fbf7..020fd2341025 100644
> --- a/drivers/md/dm-verity-target.c
> +++ b/drivers/md/dm-verity-target.c
> @@ -1399,8 +1399,8 @@ static int verity_ctr(struct dm_target *ti, unsigned argc, char **argv)
>  		goto bad;
>  	}
>  
> -	/* WQ_UNBOUND greatly improves performance when running on ramdisk */
> -	wq_flags = WQ_MEM_RECLAIM | WQ_UNBOUND;
> +	wq_flags = WQ_MEM_RECLAIM;
> +
>  	/*
>  	 * Using WQ_HIGHPRI improves throughput and completion latency by
>  	 * reducing wait times when reading from a dm-verity device.

Hi

If you remove WQ_UNBOUND, you should also change the last argument of 
alloc_workqueue from num_online_cpus() to either 0 or 1. Try both 0 and 1 
and tell us which performs better.

Mikulas