[lttng-dev] Question about performance and tracepoint structure

[Mathieu Desnoyers]

* Amit Margalit (AMITM at il.ibm.com) wrote:
> Hi,
> 
> This may be a bit of a noob question, and I think I read about this 
> somewhere but I can't find it now.
> 
> When I declare a tracepoint that's made up of a uint8_t, followed by a 
> uint16_t followed by a uint32_t, for example, there are 2 ways to handle 
> this - packed or unpacked.
> 
> If we're doing packed, then we have to include some code that does the 
> packing properly as alignment issues can creep up. This code causes us to 
> incur some CPU cycles at that point.

it's not code. it's just that cpus are slightly slower at doing
unaligned accesses than aligned accesses.

> If we're doing unpacked, then we incur some cycles on passing the event to 
> the sessiond, and sessiond suffers by having to write down some extra 
> bytes, which is less efficient.

yes

> 
> Either way, I think it means I can gain some minor performance 
> improvements if I reorder my tracepoint arguments to a more efficient 
> order - alignment-wise.

Well, yes. You will want to reorder the tracepoint "TP_FIELDS" so the
largest fields come first, followed by the smaller fields.

The order of arguments (TP_ARGS) does not matter.

Thanks,

Mathieu

> 
> Am I barking up the wrong tree here?
> 
> 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