LTTng 0.27, vmap-less buffering and splice()

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

Hi,

I have reworked the underlying buffering mechanism LTTng uses in the
past week. I took "relay", changed its vmap() buffers for my own linked
list of buffers (not vmaped), made read/write wrappers which support
writing data larger than a page size and writing across page boundaries,
and finally managed to create a splice_read file operation which
supports that. I finally changed lttd to make it use splice() instead of
a mmap() and... it worked! :-) (after a bit a debugging, clearly)

This is all in LTTng 0.27 and ltt-control 0.53 (for the lttd part).
As this is an important change, testing is very welcome. If you are
interested in looking in the inner details of the buffering mechanism I
just did, you might also want to enable the "check for random buffer
access" option in the menuconfig. It will generate warnings when offset
accesses more than a page away from the previous done is requested by
the client. Cases such as large data write and reentrancy over the
tracer code will generate a few "cache misses", but it's supposed to be
rare, and therefore not a performance concern. This self-checking
feature has proven to be very useful in the early development stages,
and I think it's also useful if any other tracer client want to use it :
it helps finding lack of reference locality a tracer client might have.

I am also very interested in getting numbers comparing the performance
of the new buffering infrastructure with the previous one. Having much
less TLB impact should improve performance.

It's all available in the git tree :
http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=summary

And the userspace packages available at http://ltt.polymtl.ca (see the
"QUICKSTART")

Cheers,

Mathieu

-- 
Mathieu Desnoyers
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