On 12/08/2014 05:57 PM, Jens Axboe wrote:
On 12/06/2014 11:30 PM, Eric Wheeler wrote:
On Sat, 6 Dec 2014, Jens Axboe wrote:
On 12/06/2014 03:40 PM, Eric Wheeler wrote:
On Fri, 5 Dec 2014, Mike Snitzer wrote:
I do wonder what the performance impact is on this for dm. Have you
tried a (worst case) test of writing blocks that are zero filled, but
with the last byte not being a zero?
The additional overhead of worst-case should be (nearly) equal to the
simplest test case of dd if=/dev/zero of=/dev/thinp/vol. In my testing
that was 1.4GB/s within KVM on an Intel Xeon(R) CPU E3-1230 V2 @
3.30GHz.
That seems way too slow for checking if it's zero or not... Memory
bandwidth should be way higher than that. The line above, was that
what you ran? How does it look with bs=4k or higher?
In userspace I can get ~12GB/s, so I think the algorithm is sound.
dd might not be the right tool for this.
It's straight forward looping through the vectors, it can't really work
any other way. But we need to figure out why it's so slow...
May be a premature optimization without any supporting data, but as this
will be a frequently running loop, it is worth optimizing.
Two tips:
1. Is the compiler unrolling loops?
Using inline bvec_kunmap_irq in the loop may prevent the compiler from
doing so.
Try breaking the loop when *parch is non zero and calling it outside of
loop only when i >= count.
2. The function is doing quite a lot of jumping around making CPU
pipeline mostly useless.
Try using kernel's built-in memcmp, which I expect to be optimized, and
compare with a zero-page.
Perhaps doing few useless bit-or ops for every write would be more
effective than this constant jumping.
-- Martian
read : io=12233MB, bw=1432.7MB/s, iops=22922, runt= 8539msec
Can you suggest the right fio commandline to test sequential writes if
all zeros? I tried --zero_buffers but couldn't get it to write zeros,
writes kept going to disk.
zero_buffers=1
scramble_buffers=0
should get you all zeroes.
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