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Re: Some newbie questions




On 08/29/2016 09:26 PM, David Smith wrote:
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?


With the new patch, I get

ERROR: probe overhead exceeded threshold
ERROR: probe overhead exceeded threshold
WARNING: Number of errors: 2, skipped probes: 0
WARNING: /home/avi/systemtap.f23/bin/staprun exited with status: 1
Pass 5: run failed.  [man error::pass5]

with -g --suppress-time-limits, I get the usual spinlock badness.



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