[ adding Shaohua ]
On Fri, Jul 13, 2012 at 3:31 AM, majianpeng <majianpeng@xxxxxxxxx> 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.
So this is a pretty obvious tradeoff of increased latency for improved
throughput. Any idea how much this change affects latency?
Especially in the fast device case?
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