On 6/19/06, Rob Gardner <rob.gardner@xxxxxx> wrote:

Stdio is there for a reason; Doing lots of raw I/O using
very small buffers is highly inefficient.

You misunderstand me. :-) I meant to write out (via DMA) several
pages at a time, straight from the HV trace buffers. The default tbuf
size in xentrace is 20 pages, so if (as the plan is) xentrace would be
notified when it would be half full, we could easily write out 10
pages in one transaction. The tbuf size could be increased if DMA
setup/teardown overhead were an issue on that scale.
You're right, for traces that fit in the file cache, buffering is a
big win. The copy overhead is negligible, writes to disk are more
efficient, and the data will be in the file cache for reading for
subsequent analysis. But for traces that won't fit in the file cache,
the best thing would be to get them to disk with as little copying and
cache-trashing as possible.
Some of my recent traces have been on the order of 10 gigabytes. I
haven't done much to modify xentrace, because I'm not worried about
the trace overhead at this point. But I've had to pull some tricks to
get my analysis tools to run in anything like a reasonable amount of
time.
-George
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