On Wed, Jan 23, 2013 at 12:50:26PM +0100, Luigi Rizzo wrote: > On Wed, Jan 23, 2013 at 12:10:55PM +0100, Stefan Hajnoczi wrote: > > On Tue, Jan 22, 2013 at 08:12:15AM +0100, Luigi Rizzo wrote: > The debugging macros (D, RD() ) are taken from our existing code, > > > > > +#define ND(fd, ... ) // debugging > > > +#define D(format, ...) \ > ... > > > +/* rate limited, lps indicates how many per second */ > > > +#define RD(lps, format, ...) \ > ... > > > Have you seen docs/tracing.txt? It can do fprintf(stderr) but also > > SystemTap/DTrace and a simple built-in binary tracer. > > will look at it. These debugging macros are the same we use in other > netmap code so i'd prefer to keep them.
Okay. Feel free to leave them. > > > +// a fast copy routine only for multiples of 64 bytes, non overlapped. > > > +static inline void > > > +pkt_copy(const void *_src, void *_dst, int l) > ... > > > + *dst++ = *src++; > > > + } > > > +} > > > > I wonder how different FreeBSD bcopy() is from glibc memcpy() and if the > > optimization is even a win. The glibc code is probably hand-written > > assembly that CPU vendors have contributed for specific CPU model > > families. > > > > Did you compare glibc memcpy() against pkt_copy()? > > I haven't tried in detail on glibc but will run some tests. In any > case not all systems have glibc, and on FreeBSD this pkt_copy was > a significant win for small packets (saving some 20ns each; of > course this counts only when you approach the 10 Mpps range, which > is what you get with netmap, and of course when data is in cache). > > One reason pkt_copy gains something is that if it can assume there > is extra space in the buffer, it can work on large chunks avoiding the extra > jumps and instructions for the remaining 1-2-4 bytes. I'd like to drop this code or at least make it FreeBSD-specific since there's no guarantee that this is a good idea on any other libc. I'm even doubtful that it's always a win on FreeBSD. You have a threshold to fall back to bcopy() and who knows what the "best" value for various CPUs is. Stefan
