[lttng-dev] memory usage depending on number and size of subbuffers

[Sébastien Barthélémy]

Hi all,

LTTng uses more memory than I expected. On the simple experiment below one
can see that the "cached" memory increases by

subbuf_size*num_cpu*(num_subbuf+1)

That's not a big deal, but I did not expect the "+1".

Does somebody know where it comes from?


-- Regards, Sebastien


12M == 2M*2*(2+1)
24M == 4M*2*(2+1)
20M == 2M*2*(4+1)
40M == 4M*2*(4+1)

# grep processor /proc/cpuinfo
processor       : 0
processor       : 1

# lttng create
Session auto-20140416-180246 created.
Traces will be written in /root/lttng-traces/auto-20140416-180246

# free -m
             total       used       free     shared    buffers     cached
Mem:          1002        670        332          0         22        250
-/+ buffers/cache:        397        605
Swap:            0          0          0

# lttng enable-channel channel0 --num-subbuf 2 --subbuf-size 2M -u
UST channel channel0 enabled for session auto-20140416-180246

# free -m
             total       used       free     shared    buffers     cached
Mem:          1002        682        320          0         22        262
-/+ buffers/cache:        397        605
Swap:            0          0          0

# lttng destroy
Session auto-20140416-180246 destroyed

# lttng create
Session auto-20140416-180317 created.
Traces will be written in /root/lttng-traces/auto-20140416-180317

# free -m
             total       used       free     shared    buffers     cached
Mem:          1002        670        332          0         22        250
-/+ buffers/cache:        397        605
Swap:            0          0          0

# lttng enable-channel channel0 --num-subbuf 2 --subbuf-size 4M -u
UST channel channel0 enabled for session auto-20140416-180317

# free -m
             total       used       free     shared    buffers     cached
Mem:          1002        694        307          0         22        274
-/+ buffers/cache:        397        605
Swap:            0          0          0

# lttng destroy
Session auto-20140416-180317 destroyed

# lttng create
Session auto-20140416-180339 created.
Traces will be written in /root/lttng-traces/auto-20140416-180339

# free -m
             total       used       free     shared    buffers     cached
Mem:          1002        670        332          0         22        250
-/+ buffers/cache:        397        605
Swap:            0          0          0

# lttng enable-channel channel0 --num-subbuf 4 --subbuf-size 2M -u
UST channel channel0 enabled for session auto-20140416-180339

# free -m
             total       used       free     shared    buffers     cached
Mem:          1002        690        311          0         22        270
-/+ buffers/cache:        397        605
Swap:            0          0          0

# lttng destroy
Session auto-20140416-180339 destroyed

# lttng create
Session auto-20140416-180406 created.
Traces will be written in /root/lttng-traces/auto-20140416-180406

# free -m
             total       used       free     shared    buffers     cached
Mem:          1002        670        332          0         22        250
-/+ buffers/cache:        397        605
Swap:            0          0          0

# lttng enable-channel channel0 --num-subbuf 4 --subbuf-size 4M -u
UST channel channel0 enabled for session auto-20140416-180406

# free -m
             total       used       free     shared    buffers     cached
Mem:          1002        710        292          0         22        290
-/+ buffers/cache:        397        605
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