[lttng-dev] Interplay of enable-channel options

[Daniel.Thibault@drdc-rddc.gc.ca (Thibault, Daniel)]

Harking back to Mathieu's post of 3 May 2013 10:42:22, I'd like to see if I understand the options correctly.  I'm pretty sure I don't.

When a channel has been set to use per-UID buffering, if an event-producing user-space process dies suddenly it may create a "hole" in one of the channel's sub-buffers, temporarily preventing the consumer from getting access to the sub-buffer.  Once the other event producers have filled the remaining sub-buffers, the session daemon will try to re-use the "hung" sub-buffer, at which point it will be discarded.  Unrelated events in the sub-buffer will be lost.

If the channel is also set to --discard, does this mean the session daemon will not flush the hung sub-buffer?  This would mean the consumer daemon waits at the holed sub-buffer and never gets past it.

What if --read-timer has been set to a non-zero value?  Will the holed sub-buffer be flushed at that point in time?  In this case the producers may not even have time to fill the remaining sub-buffers.

How can the session daemon create a hole in a sub-buffer, anyway?  Doesn't it get all of the event's description and payload in its own working memory before starting to write it out to the sub-buffer?
 
Daniel U. Thibault
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