Re: [PATCH V1] raidd5:Only move IO_THRESHOLD stripes from delay_list to hold_list once.

[NeilBrown <neilb@suse.de>]

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On Fri, 13 Jul 2012 18:31:11 +0800 majianpeng <majianpeng@gmail.com> wrote:

> To improve write perfomance by decreasing the preread stripe,only move
> IO_THRESHOLD stripes from delay_list to hold_list once.
> 
> Using the follow command:
> dd if=/dev/zero of=/dev/md0 bs=2M count=52100.
> 
> At default condition: speed is 95MB/s.
> At the condition of preread_bypass_threshold was equal zero:speed is 105MB/s.
> Using this patch:speed is 123MB/s.
> 
> If preread_bypass_threshold was zero,the performance will be better,but
> not better than this patch.
> I think maybe two reason:
> 1:If bio is REQ_SYNC
> 2:In function __get_priority_stripe():
> >> } else if (!list_empty(&conf->hold_list) &&
> >>		   ((conf->bypass_threshold &&
> >>		     conf->bypass_count > conf->bypass_threshold) ||
> >>		    atomic_read(&conf->pending_full_writes) == 0)) {
> Preread_bypass_threshold is one condition of getting stripe from
> hold_list.So only control the number of hold_list can get better
> performance.
> 
> Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
> ---
>  drivers/md/raid5.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 04348d7..a6749bb 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -3662,6 +3662,7 @@ finish:
>  
>  static void raid5_activate_delayed(struct r5conf *conf)
>  {
> +	int count = 0;
>  	if (atomic_read(&conf->preread_active_stripes) < IO_THRESHOLD) {
>  		while (!list_empty(&conf->delayed_list)) {
>  			struct list_head *l = conf->delayed_list.next;
> @@ -3672,6 +3673,8 @@ static void raid5_activate_delayed(struct r5conf *conf)
>  			if (!test_and_set_bit(STRIPE_PREREAD_ACTIVE, &sh->state))
>  				atomic_inc(&conf->preread_active_stripes);
>  			list_add_tail(&sh->lru, &conf->hold_list);
> +			if (++count >= IO_THRESHOLD)
> +				break;
>  		}
>  	}
>  }


I tried this patch - against my current for-next tree - on my own modest
hardware and could not measure any difference in write throughput.

Maybe some other patch has fixed something.

However it is still reading a lot during a write-only test and that is not
ideal.  It would be nice if we could arrange that it didn't read at all.

NeilBRown

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