RELENG_10 performance regression (was Re: 35-40% performance drop releng9 vs releng10 openvpn

[Adrian Chadd]

On 21 March 2015 at 08:52, John Baldwin <jhb at freebsd.org> wrote:
> On 3/20/15 8:46 PM, Mike Tancsa wrote:
>> On 3/20/2015 8:15 PM, Konstantin Belousov wrote:
>>>>
>>>> For the purpose of devfs, does it make sense to bump timestamps like
>>>> normal filesystems for each read/write operation?  Looks like Mac OS X
>>>> will bump timestamps for each operation but Debian don't.
>>>
>>> First question is, what timecounter hardware is used.  I would accept
>>> some slowdown from hardware like HPET, but it is indeed surprising
>>> if caused by TSC.
>>>
>>>
>>
>> David Wolfskill suggested trying the problem commit with
>>
>> vfs.timestamp_precision=0
>>
>> and it does indeed restore performance to what it was.  The raw dtrace
>> files are available and FlameGraphs can all be found at
>>
>> http://tancsa.com/time/
>
> Do you know why you are using the HPET instead of TSC for timestamping?
> Using the TSC can make a non-trivial performance difference since userland
> can calculate timestamps without using system calls when it is used.
> (That is not related to this case, but switching to the TSC in general is
> preferable.)
>
> There are a few generations of Intel CPUs where you can't mix deeper sleep
> states with the TSC as timecounter, but those CPUs are getting to be a bit
> older at this point.

What about various VMs?



-adrian