Hi Zach,
there's a proposed change to lttng-ust which addresses the issue you are
seeing, but it hasn't finished clearing review & testing yet.
https://review.lttng.org/c/lttng-ust/+/11006
thanks,
kienan
On 2023-10-05 18:04, Kienan Stewart via lttng-dev wrote:
Hi Zach
apologies for the delay in replying to your question.
On 2023-09-26 08:27, Kramer, Zach via lttng-dev wrote:
Hi,
I am observing a performance issue with regards to enabling events
while a session is active and was wondering if this is expected.
LTTng versions:
* lttng-tools: 2.13.9
* lttng-ust: 2.13.6
Steps to reproduce:
1. Ensure many userspace tracepoints are available in `lttng list -u`
e.g. 100
2. Create a new session
3. Start session
4. Enable new events on session
The time it takes to enable each new event has increasing cost e.g.
1. Event 1: 1ms
2. Event 100: 15ms
3. Event 1000: 150ms
4. àin total about 1.5 minutes to enable 1000 events
If either:
1. No userspace tracepoints are available
2. Or session is not started until /after /the events are enabled
Then the time it takes to enable new events is constant (e.g. 1ms).
Below is a bash script demonstrating this behavior:
# Pre-requisite: have many userspace tracepoints available
lttng create foo
lttng enable-channel -u -s foo bar
lttng start foo
total_t1=$(date +%s%3N);
for i in {1..100}
do
t1=$(date +%s%3N);
lttng enable-event -u lttng_iter_$i -s foo -c bar > /dev/null
t2=$(date +%s%3N);
echo "Event #$i took $((t2-t1)) ms"
done
total_t2=$(date +%s%3N);
echo "----------------------------------------------------"
echo "Enabling events on active session took $((total_t2-total_t1)) ms"
echo "----------------------------------------------------"
lttng destroy foo
lttng create foo
lttng enable-channel -u -s foo bar
total_t1=$(date +%s%3N);
for i in {1..100}
do
t1=$(date +%s%3N);
lttng enable-event -u lttng_iter_$i -s foo -c bar > /dev/null
t2=$(date +%s%3N);
echo "Event #$i took $((t2-t1)) ms"
done
total_t2=$(date +%s%3N);
echo "----------------------------------------------------"
echo "Enabling events on inactive session took $((total_t2-total_t1)) ms"
echo "----------------------------------------------------"
lttng destroy foo
Is this reproducible for you? Any insight is appreciated.
I'm able to reproduce what you're seeing, and it's not completely
unexpected based on my understanding of the architecture of lttng.
When a session is active and an event rule is enabled/disabled, the
sessiond will notify each of the registered applications of the event
rule state changes. This communication is going through either unix or
tcp sockets, and that portion of the protocol seems to struggle with
many small changes as the protocol to communicate the changes doesn't
support batching.
Enabling multiple events in a single call reduces some of the overhead,
but the changes will still be communicated serially. This means a single
call activating thousands events can still take a long time to process
(eg. `lttng enable-event -s foo -c bar -u tp0,tp1,...,tpN`)
Using glob patterns or `--all` will be significantly faster as the UST
applications can digest those types of event rule changes with a single
set of messages. In cases where you want most but not all events,
flipping the logic to enable "*" but add events or patterns to omit with
"--exclude" is going to be a better strategy.
Do you have cases where you need to activate many of events but which
couldn't be covered by using glob patterns and/or exclusions?
thanks,
kienan
Many thanks,
Zach
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