Re: PATCH/RFC: Bring lin-lwp performance back to the real world

Daniel Jacobowitz
2002-11-22T05:29:45Z

On Thu, Nov 21, 2002 at 11:51:01PM -0500, Andrew Cagney wrote:
> > - It's such a wonderful bandaid that a lot of the badly needed
> > cleanups may lose momentum.
>
> ``Don't you worry about that'' :-)
>
> A back of envelope calculation shows:
>
> >Plucking a ``random'' memory location out of thin air:
> >
> >cagney@torrens$ grep 0x40040ea0 gdb.strace | grep ptrace | wc -l
> > 7038
>
> For <160 stops, GDB fetched a thread-db buffer 7000 times. So, GDB is
> fetching an identical buffer 7000/160 ~= 50 times for every stop!
>
> >cagney@torrens$ expr 7038 \* 250
> >1759500
>
> 250? I happen to know that the buffer is ~1000 bytes long which gives
> us 250 (1000/sizeof(long) ptrace() calls for every buffer transfer.
>
> The patch addresses the second problem, but not the first. While it
> releaves a bit of steam, there is still plenty of oportunity to further
> ramp up the performance by several more orders of magnitude.
>
> (BTW, this sort of brain-deadness on the part of GDB explains why other
> minor performance tweeks had zero benefit :-)
>
> ----
>
> Note that the other /proc trickery is in linux-proc.c. Should this be
> there?

Hrm, possibly. I needed to create linux-nat.c anyway (I'll need it for
some things that are definitely not /proc related) but I could be
persuaded either way on linux_proc_xfer_memory. It's not focused on
the "proc" bit as much as the "xfer" bit, but it's definitely using
/proc. If you prefer I'll move it, and save linux-nat.c for another
patch.

Thanks for the pointer to pread. I don't know how far back it's
available... looks like 1997-09-30 in glibc. Could just assume
pread64 and force -D_LARGEFILE64_SOURCE, I think - it's been there in
all of glibc2 and libc4/libc5 are pretty darned niche now. Could
autoconf for it instead, easy enough...

I think I'll update the patch to autoconf check for pread64 with
appropriate compilation flags, and fall back to 32-bit lseek/read.
I'll repost it after I decide what to do about linux-proc vs linux-nat,
and any other comments come in (i.e. not tonight!).

--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer