[lttng-dev] subbuffers count and size

[Vijay Anand]

Hi,

I have been trying to understand the impact of performance while logging
user space program based on the subbuffer count and size.

I have a simple program logging 1 million traces and I dont seem to see any
appreciable performance between the below two configurations.

a. default 4 subbufers 128KB each
b. 16 subbufers with 1024kB each.

Moreover I have been looking at the lttng-ust code to understand the
performance impacts from the implementation and unable to comprehend what
is documented at

http://lttng.org/docs/#doc-channel-subbuf-size-vs-subbuf-count

I see that channel_backend_init, _shm_object_table_alloc_shm seems to
allocate one big shared memory chunk which again is subdivided into
subbuffers with each subbuffer referenced using the subbufer index.

Further more lib_ring_buffer_write seems to find the subbuffer index and
write "len" number of bytes into the subbuffer index.

Could anyone please enlighten me what is the overhead involved while
switching from one subbufer to another as documented.

Regards,
Vijay
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