[lttng-dev] Use of sub-buffers - more subbuffers but smaller, or less but bigger?

[Mathieu Desnoyers]

* Amit Margalit (AMITM at il.ibm.com) wrote:
> Hello,
> 
> I'm trying to understand the trade-offs involved in choosing sub-buffer 
> number and size.
> 
> So the question is what are the benefits and the disadvantages of 
> choosing, say, 32 subbuffers of 1MB versus 8 subbuffers of 4MB, or maybe 4 
> subbuffers of 8MB?

Boundaries of subbuffers are synchronization points, so you add slightly
more overhead with smaller sub-buffers. This is an argument for having
few large sub-buffers.

Also, with periodical timer flush (useful for upcoming streaming
feature), you might want more than two sub-buffers per buffer, because
otherwise, you might run into situations where the buffer appears to be
full (and therefore events are discarded) just because the timer flush
flushes a sub-buffer before it is filled. This is an argument for not
making the number of sub-buffers _too_ small if you use switch timer.

Hoping this helps,

Mathieu

> 
> Thanks,
> 
> Amit Margalit
> IBM XIV - Storage Reinvented
> XIV-NAS Development Team
> Tel. 03-689-7774
> Fax. 03-689-7230
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-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com