Re: [PATCH v5 00/16] md/raid5: set STRIPE_SIZE as a configurable value

Song Liu
2020-07-10T16:09:53Z

On Thu, Jul 9, 2020 at 6:27 AM Yufen Yu <yuyufen@huawei.com> wrote:
>
>
>
> On 2020/7/9 7:55, Song Liu wrote:
> > On Wed, Jul 8, 2020 at 6:15 AM Yufen Yu <yuyufen@huawei.com> wrote:
> >>
> >>
> >>
> >> On 2020/7/3 7:00, Song Liu wrote:
> >>> On Thu, Jul 2, 2020 at 5:05 AM Yufen Yu <yuyufen@huawei.com> wrote:
> >>>>
> >>>> Hi, all
> >>>>
> >>>> For now, STRIPE_SIZE is equal to the value of PAGE_SIZE. That means, RAID5
> >>>> will issue each bio to disk at least 64KB when PAGE_SIZE is 64KB in arm64.
> >>>> However, filesystem usually issue bio in the unit of 4KB. Then, RAID5 may
> >>>> waste resource of disk bandwidth.
> >>>>
> >>>> To solve the problem, this patchset try to set stripe_size as a configuare
> >>>> value. The default value is 4096. We will add a new sysfs entry and set it
> >>>> by writing a new value, likely:
> >>>>
> >>>> echo 16384 > /sys/block/md1/md/stripe_size
> >>>
> >>> Higher level question: do we need to support page size that is NOT 4kB
> >>> times power
> >>> of 2? Meaning, do we need to support 12kB, 20kB, 24kB, etc. If we only
> >>> supports, 4kB,
> >>> 8kB, 16kB, 32kB, etc. some of the logic can be simpler.
> >>
> >> Yeah, I think we just support 4kb, 8kb, 16kb, 32kb... is enough.
> >> But Sorry that I don't know what logic can be simpler in current implementation.
> >> I mean it also need to allocate page, and record page offset.
> >
> > I was thinking about replacing multiplication/division with bit
> > operations (shift left/right).
> > But I am not very sure how much that matters in modern ARM CPUs. Would you mind
> > running some benchmarks with this?
>
> To test multiplication/division and bit operation, I write a simple test case:
>
> $ cat normal.c
>
> int page_size = 65536;
> int stripe_size = 32768; //32KB
>
> int main(int argc, char *argv[])
> {
> int i, j, count;
> int page, offset;
>
> if (argc != 2)
> return -1;
>
> count = atol(argv[1]);
>
> for (i = 0; i < count; i++) {
> for (j = 0; j < 4; j++) {
> page = page_size / stripe_size;
> offset = j * stripe_size;
> }
> }
> }
>
> $ cat shift.c
>
> int page_shift = 16; //64KB
> int stripe_shift = 15; //32KB
>
> int main(int argc, char *argv[])
> {
> int i, j, count;
> int page, offset;
>
> if (argc != 2)
> return -1;
>
> count = atol(argv[1]);
>
> for (i = 0; i < count; i++) {
> for (j = 0; j < 4; j++) {
> page = 1 << (page_shift - stripe_shift);
> offset = j << stripe_shift;
> }
> }
> }
>
> Test them on a arm64 server, the result show there is a minor
> performance gap between multiplication/division and shift operation.
>
> [root@localhost shift]# time ./normal 104857600
>
> real 0m1.199s
> user 0m1.198s
> sys 0m0.000s
>
> [root@localhost shift]# time ./shift 104857600
>
> real 0m1.166s
> user 0m1.166s
> sys 0m0.000s
>
> For our implementation, page address and page offset are just computed
> when allocate stripe_size. After that, we just use the recorded value
> in sh->dev[i].page and sh->dev[i].offset. So, I think current implementation
> may not cause much overhead.

Sounds good. Let's keep this part as-is.

Thanks,
Song