Date: Mon, 17 Feb 1997 01:04:24 +1100 From: Bruce Evans <bde@zeta.org.au> To: ache@nagual.ru, peter@spinner.dialix.com Cc: bde@zeta.org.au, cvs-all@freefall.freebsd.org, CVS-committers@freefall.freebsd.org, cvs-sys@freefall.freebsd.org, mpp@freefall.freebsd.org Subject: Re: cvs commit: src/sys/sys types.h Message-ID: <199702161404.BAA20251@godzilla.zeta.org.au>
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>I don't understand why rounding is even needed here (maybe to make less
32-bit rounding is better for bzero, copyout, etc, and doesn't cost more
storage. 256-bit rounding avoids slow startup but isn't space-efficient
since we're allocating 6 times that. Anyway, I think it would be
better to use an auto buffer for small allocations even p_selbits has
been allocated. This would have better locality. Most applications
wouldn't need a malloced buffer. Half of implementation:
fd_mask selbits[6 * OLD_FD_SETSIZE / NFDBITS];
...
ni = bits_to_copyin(); /* multiple of NFDBITS */
if (ni <= OLD_FD_SETSIZE)
use_selbits();
else
use_malloced_bits();
>*** sys_generic.c.orig Tue Jan 14 23:11:28 1997
>--- sys_generic.c Sun Feb 16 16:00:48 1997
>***************
>*** 508,513 ****
>--- 508,515 ----
> return (error);
> }
>
>+ #define FD_ROUNDSIZE 256
>+
Change to something like (2048 / 6) = 341. Now that's an odd size :-).
>--- 562,580 ----
> p->p_selbits_size);
> }
>
> /* The amount of space we need to copyin/copyout */
> ni = howmany(uap->nd, NFDBITS) * sizeof(fd_mask);
>+
>+ if (ni > 8) { /* 64 bits - arbitary guess at tradeoff */
>+ /*
>+ * Clear the entire return-to-user buffer space
>+ */
>+ bzero (p->p_selbits + 3 * p->p_selbits_size,
>+ p->p_selbits_size * 3);
>+ } else {
>+ for (i = 0; i < 3; i++)
>+ bzero(obits[i], ni);
>+ }
Don't use this except possibly for very large ni together with unusual
combinations of fds. Put 3 contiguous copyout buffers first (readfds,
writefds, exexptfds), then 3 copyin buffers, and bzero 1*ni, 2*ni or
3*ni bits as necessary (exceptfds is usually NULL so the bzero doesn't
need to cover it). The old code got this wrong mainly by spacing the
buffers with separation p->p_selbits_size instead of ni.
Bruce
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