Messages in this thread |  | | | Date | Thu, 26 Feb 2009 22:46:56 -0800 | | From | Andrew Morton <> | | Subject | Re: [PATCH][RFC] vsprintf: unify the format decoding layer for its 3 users |
| |
On Fri, 27 Feb 2009 07:19:37 +0100 Frederic Weisbecker <fweisbec@gmail.com> wrote:
> > An new optimization is making its way to ftrace. Its purpose is to > make ftrace_printk() consuming less memory and be faster. > > Written by Lai Jiangshan, the approach is to delay the formatting > job from tracing time to output time. > Currently, a call to ftrace_printk will format the whole string and > insert it into the ring buffer.
It does that? eek.
> Then you can read it on /debug/tracing/trace > file. > > The new implementation stores the address of the format string and > the binary parameters into the ring buffer, making the packet more compact > and faster to insert. > Later, when the user exports the traces, the format string is retrieved > with the binary parameters and the formatting job is eventually done. > > Here is the result of a small comparative benchmark while putting the following > ftrace_printk on the timer interrupt. ftrace_printk is the old implementation, > ftrace_bprintk is a the new one: > > ftrace_printk("This is the timer interrupt: %llu", jiffies_64); > > After some time running on low load (no X, no really active processes): > > ftrace_printk: duration average: 2044 ns, avg of bytes stored per entry: 39 > ftrace_bprintk: duration average: 1426 ns, avg of bytes stored per entry: 16 > > Higher load (started X and launched a cat running on an X console looping on > traces printing): > > ftrace_printk: duration average: 8812 ns > ftrace_bprintk: duration average: 2611 ns > > Which means the new implementation can be 70 % faster on higher load. > And it consumes lesser memory on the ring buffer. > > The curent implementation rewrites a lot of format decoding bits from > vsnprintf() function, making now 3 differents functions to maintain > in their duplicated parts of printf format decoding bits.
<looks for ftrace_bprintk() in linux-next, fails>
Why does the current ftrace_bprintk() need to hack around in (or duplicate) vprintk() internals? It's a bit grubby, but by placing an upper bound on the number of args, it could simply call vscnprintf() directly?
|  |