Hi Lakshya,
On 3/12/25 5:03 AM, Gour DEV wrote:
Hi, Kienan
I am attaching an screen recording of the behaviour I am seeing in this
mail. The behaviour is same irrespective of the device i use, sorry for
miscommunication in the npocs output (I assumed it was 32), but other than
that all outputs are same (except the hostname as there are multiple
devices with same lttng config but this memory cosumption is seen on all
the devices).
I had few question
1. Does lltng allocated all the memory it needs and mark it as dirty in ram
when any process which links/uses lttng-ust runs? (here i tried with one
process but it is same for any of my process)
I believe the shared memory for per-CPU data structures is allocated
when an instrumented application connects. There is no pre-allocation
for each possible UID on the system.
You can run your instrumented applications with `LTTNG_UST_DEBUG=1` to
see when the connection happens[1].
2. (nSubbuf * subbufSize) * (nCPUs + 1 iff snapshot mode is enabled) *
(nUIDs or nPIDs)
How do we calculate uid in the system is it all uids in the system? is it
equal to `cat /etc/passwd | wc -l` ?
nUIDs is the number of distinct UIDs running instrumented applications.
I will put my calculations according to the above estimate based on all the
channel i am creating
(4194304*4 + 262144*4 + 16384*4) * (16) * (30 if number user are equal to
`cat /etc/passwd | wc -l`)B = 7.998046875 GB approx [this is based on the
start_lttng.py please do correct me if am wrong here.]
But since there are only two users which uses lttng i think the correct
estimate would be
(4194304*4 + 262144*4 + 16384*4) * (16) * (2)B = 546MB
The estimate I gave is per-channel.
small channel: (0.015625 MiB * 4) * (16 + 1) = 1.0625 MiB per-channel
per-UID
medium channel: (0.250 MiB * 4) * (16 + 1) = 17.0 MiB per-channel per-UID
large channel: (4 MiB * 4) * (16 + 1) = 27 2MiB per-channel per-UID
Now, you said you have 0 small channels, 6 medium channels, and 16 large
channels in your session. (Note: I see your script differs from these
stated channel counts).
small: 0 * 1.0625 MiB = 0 MiB per-UID
medium: 6 * 17 MiB = 102 MiB per-UID
large: 16 * 272 MiB = 4352 MiB per-UID
And if you're running instrumented applications with 2 users:
small: 0 MiB * 2 = 0 MiB with 2 UIDs
medium: 102 MiB * 2 = 204 MiB with 2 UIDs
large: 4352 MiB * 2 = 8704 MiB with 2 UIDs
Now this is just an estimation for the per-CPU ring buffers only, and
you numbers aren't hugely off so without analyzing your specific system
it doesn't seem to be that strange to me.
If I take the number of channels I see in your script, it becomes:
small: 0 MiB with 2 UIDs
medium: 136 MiB with 2 UIDs
large: 7616 MiB with 2 UIDs
total: 7.57 GiB with 2 UIDs
Please do correct me If I am wrong calculations here.
Now, there are a few things here, according to my output lttng is using 11G
which is much more higher than the what is configured.
I have no idea what 'service start spyder' is doing. Maybe it's running
instrumented applications with an extra user that you didn't expect? I
can't help you with that aspect of your system.
The above estimated 7.57 GiB with 2 UIDs would be 11.35 GiB with 3 UIDs
so maybe?
I'd recommend you read your verbose sessiond log so see which
applications are connecting and with which UIDs.
I am attaching the lttng status and the file which is uses to create the
lttng sessions.
Thank You.
In any case, the information you have given to date hasn't demonstrated
to me in a tangible manner that you are seeing a difference related to
the version of LTTng being used.
thanks,
kienan
[1]: https://lttng.org/man/3/lttng-ust/v2.13/#doc-_environment_variables
