[lttng-dev] Question about performance and tracepoint structure

[Mathieu Desnoyers]

* Amit Margalit (AMITM at il.ibm.com) wrote:
> Thanks, that helps a lot.
> 
> By the way - how do strings fit in this? Is it best that they are last? 
> Maybe best they are first being the largest?

last. Those are really an array of characters, terminated by '\0',
therefore the alignment is that of a character (1-byte).

Thanks,

Mathieu

> 
> Amit Margalit
> IBM XIV - Storage Reinvented
> XIV-NAS Development Team
> Tel. 03-689-7774
> Fax. 03-689-7230
> 
> 
> 
> From:   Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> To:     Amit Margalit/Israel/IBM at IBMIL
> Cc:     lttng-dev at lists.lttng.org
> Date:   07/10/2013 08:25 PM
> Subject:        Re: [lttng-dev] Question about performance and tracepoint 
> structure
> 
> 
> 
> * 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
> > _______________________________________________
> > lttng-dev mailing list
> > lttng-dev at lists.lttng.org
> > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
> 
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
> 
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com