Some newbie questions

[David Smith]

On 08/28/2016 08:29 AM, Avi Kivity wrote:
> 
> 
> On 08/26/2016 11:07 PM, David Smith wrote:
>> On 08/26/2016 01:39 PM, Avi Kivity wrote:
>>> On 08/25/2016 08:37 PM, David Smith wrote:
>>>> On 08/25/2016 11:21 AM, Avi Kivity wrote:
>>>>> Hi,
>>>>>
>>>>> Should I wait for ongoing improvement here, or shall I look elsewhere
>>>>> for my tracing needs?
>>>>>
>>>>> It would be a pity (for me) if I have to find another solution,
>>>>> because
>>>>> systemtap has all the expressiveness and integration I need.  But
>>>>> it has
>>>>> a dramatic impact on my application runtime.
>>>>>
>>>>> I was able to extract some useful data with perf probe/perf record,
>>>>> but
>>>>> as soon as I need to qualify a probe point with runtime information,
>>>>> perf falls short.
>>>> As Frank mentioned in a previous email, it might be possible for us to
>>>> switch to using straight kprobes instead of syscall tracing to handle
>>>> mmap tracing. In your use case of calling epoll_wait() lots of times
>>>> per
>>>> second, that might be a *big* win.
>>>>
>>>> I'll see what can be done to add that feature.
>>>>
>>> Thanks a lot.  I'll be happy to test patches.
>> OK, since you asked...
>>
>> Here's a patch I'm testing that tries to do prefiltering when a syscall
>> occurs, so we don't have to take that lock.
>>
>> Please rebuild with it, and let me know if it (a) works, and (b) has
>> lower overhead in your situation.
>>
> 
> With an unloaded system, systemtap almost vanishes from the profile. 
> This is on a 2s24c48t system, running epoll_pwait() and polling on user
> memory locations in a tight loop.

... stuff deleted ...

After looking into it, Josh was right in thinking I had the test
backwards. Can you try the following patch (after reverting the last
patch) and let us know what you find?

-- 
David Smith
dsmith@redhat.com
Red Hat
http://www.redhat.com
256.217.0141 (direct)
256.837.0057 (fax)
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