[lttng-dev] What is the cost of user-space tracepoint() ?

[ChitlurSrinivasa, Venkatesh]

Mathieu,

I tried to send this email to lttng-dev@ but I didn't get any response. So I am sending this directly to you. I greatly appreciate your response.

?

We are planning to use LTTng UST as it supports lot of interesting features, but have some performance concerns (as compared with our in-house tracing tool). Please point me to the latest benchmark tests and performance results. On CPU Intel Xeon E5-2680 v2 @ 2.80GHz?, running Linux 3.6.11? and lttng 2.4.1,  I am getting about 927 cycles (9270692144 cycles for 10000000? iterations). This seems to be lot higher than the documented results. In the paper https://lttng.org/files/papers/desnoyers.pdf  the average cost of tracepoint() with older ltt-usertrace-fast tracepoint is 297 cycles. Another link http://lttng.org/files/thesis/desnoyers-thesis-defense-2009-12-e1.pdf says cache hot tracepoint() cost is 238 cycles.?

I noticed that the tracepoint is making clock_gettime() and sched_getcpu() system calls. With Linux kernel v3.6.11 and libc v2.13-38+deb7u3, I see that these system calls are not going through VDSO and hence costing more. I tried to add wrapper functions for these system calls to call __vdso_clock_gettime and __vdso_getcpu as upgrading libc was not an option. With this change the cost of tracepoint() recording one integer dropped to 795 cycles (= 284 nsec on Intel(R) Xeon(R) CPU E5-2680 v2 @ 2.80GHz). Still this number seems to be higher than earlier published numbers.

VBabu
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