Re: [PATCH] gettime: minimize integer division

[Jens Axboe <axboe@kernel.dk>]

On 2012-12-21 22:28, Sam Bradshaw (sbradshaw) wrote:
> 
>> -----Original Message-----
>> From: Jens Axboe [mailto:axboe@kernel.dk]
>> Sent: Friday, December 21, 2012 7:33 AM
>> To: Sam Bradshaw (sbradshaw)
>> Cc: fio@vger.kernel.org
>> Subject: Re: [PATCH] gettime: minimize integer division
>>
>> On 2012-12-20 20:23, Sam Bradshaw wrote:
>>>
>>>
>>> Something like this might work, though that amount of logic may
>>> be equivalent in terms of cycles to the divide.
>>
>> So I took a look at it. The costly bit is the division by
>> cycles_per_usec, which the compiler has no other option than turn into a
>> divq. The modulo and divide by 1M can be turned into something more
>> clever, basically shifts and imull.
>>
>> So how about the below? It turns the divq into multiplication and
>> division by 10M, which should be considerably less expensive. Can you
>> test and see how that works for you?
> 
> That works much better.  Several % lower execution time in fio_gettime().

Goodie

> IOPs look the same in my synthetic test but I'm not sure that's relevant;
> (it probably just needs some more tweaking).

It'd probably need 3-4M IOPS from a single thread to have a big impact.
But reduced CPU is leftover CPU for doing actual IO, so always a good
thing.

And just as important, did the timing look correct?

> I'll keep hunting for other hot spots.

Thanks!

-- 
Jens Axboe