Subject: [PATCH] arm: dts: exynos5: Remove multi core timer

From: [email-masked] (Tomasz Figa)
Date: Fri, 16 May 2014 01:25:22 +0200

On 16.05.2014 01:18, David Riley wrote:
> On Thu, May 15, 2014 at 4:03 PM, Chirantan Ekbote
> <[email-masked]> wrote:
>> Hi Tomasz,
>>
>> On Thu, May 15, 2014 at 3:44 PM, Doug Anderson <[email-masked]> wrote:
>>> Tomasz,
>>>
>>> On Thu, May 15, 2014 at 3:13 PM, Tomasz Figa <[email-masked]> wrote:
>>>>> NOTE: if for some reason we need to keep the MCT around, we're
>>>>> definitely going to need to account for the fact that tweaking it
>>>>> affects the arch timer.  ...and having the arch timer is really nice
>>>>> since:
>>>>
>>>> [Let me reorder the points below to make it easier to comment:]
>>>>
>>>>> * it's faster to access.
>>>>> * it is accessible from userspace for really fast access.
>>>>
>>>> Do you have some data on whether it is a significant difference,
>>>> especially considering real use cases?
>>>
>>> I know that Chrome makes _a lot_ of calls to gettimeofday() for
>>> profiling purposes, enough that it showed up on benchmarks.  In fact,
>>> we made a change to the MCT to make accesses faster and there's a
>>> small mention of the benchmarking that was done at:
>>>
>>> https://chromium-review.googlesource.com/#/c/32190/
>>>
>>> ...that change probably should be sent upstream, actually.
>>>
>>> I'll let Chirantan comment on how much faster arch timers were.
>>> ...and I think David Riley (also CCed now) may be able to comment on
>>> the benefits of userspace timers.
>>>
>>
>> When I profiled gettimeofday() calls, they were about 50 - 60% faster
>> with the arch timers compared to the mct.
> 
> When I profiled gettimeofday(), the standard systems call version took
> about 2.5x longer than through a vDSO interface.

Sounds like we should invent a new kind of jokes, starting with "When I
profiled gettimeofday()". ;)

Just kidding.

The raw improvement looks quite good, but what I'm more concerned about
is whether this has any significant effect on real use cases. As Doug
mentioned, Chrome makes a lot of calls to gettimeofday(), but he also
mentioned that this is for profiling purposes. Is performance of
gettimeofday() really that crucial in this case? Are there any other use
cases when this improvement is significant?

Anyway, I'm by no means opposed to switching to arch timers. They
provide a well designed, generic interface and drivers shared by
multiple platforms, which means more code sharing and possibly more eyes
looking at the code, which is always good. However if they don't support
low power states correctly, we can't just remove MCT.

Best regards,
Tomasz