Re: [RFC PATCH lttng-modules] LTTng logger ABI

[Karim Yaghmour <karim.yaghmour@opersys.com>]

On 14-02-17 11:21 AM, Mathieu Desnoyers wrote:
> I'm seeing here that the event in trace_marker is limited to 1 page (it can
> cross a page boundary, so ftrace deals with 2 pages at most). It makes sense,
> since Ftrace's buffers are split into pages.
> 
> Its truncation happens here:
> 
>         if (cnt > TRACE_BUF_SIZE)
>                 cnt = TRACE_BUF_SIZE;

Good to know. So at least we know ftrace truncates by default.

> The minimum size of sub-buffers (trace packets) LTTng can allocate is a page.
> However, the available space for event records is slightly less, since we have
> a packet header at the beginning of the packet. Since the packet header can
> be extended between LTTng versions, it's hard to say. The space can be
> slightly less if event contexts are enabled (this can be done dynamically
> at runtime, per-buffer).
> 
> Thoughts ?

I'd be tempted to say that anything that big should be good enough. But
from the sounds of it, even doing strlen's in user-space wouldn't be
good enough to ensure that the data fits in lttng buffer. I can't say
that I have a good solution here. Ideally there should be some form of
automatic truncation somewhere without having to go through the expense
of strlens or the likes.

-- 
Karim Yaghmour
CEO - Opersys inc. / www.opersys.com
http://twitter.com/karimyaghmour