Re: [flashrom] [PATCH] Reduce realloc syscall overhead for ft2232 and bitbang

[Sean Nelson]

On 11/25/2009 7:31 AM, Paul Fox wrote:
> carl-daniel wrote:
> > Reduce realloc syscall overhead for FT2232 and bitbang.
> >
> > FT2232 ran realloc() for every executed command. Start with a big enough
> >  buffer and don't touch buffer size unless it needs to grow.
> >  Bitbang was slightly better: It only ran realloc() if buffer size
> >  changed. Still, the solution above improves performance and reliability.
>
> this is fine, but i'm curious -- did you measure a performance
> change, or is this "by inspection"?  it's never been my
> impression that historically realloc would be slow, unless the
> buffer is growing -- in which case there's no choice.  and my perhaps
> mistaken assumption was that realloc() would usually not shrink a
> buffer.
>
> paul
> > Signed-off-by: Carl-Daniel Hailfinger
> > Index: flashrom-realloc_overhead/bitbang_spi.c
> >
> > --- flashrom-realloc_overhead/bitbang_spi.c	(Revision 777)
> >  +++ flashrom-realloc_overhead/bitbang_spi.c	(Arbeitskopie)
> >  @@ -87,14 +87,16 @@
> >    static unsigned char *bufout = NULL;
> >    static unsigned char *bufin = NULL;
> >    static int oldbufsize = 0;
> > int bufsize = max(writecnt + readcnt, 260);
> >
> > int bufsize;
> > int i;
> >
> > /* Arbitrary size limitation here. We're only constrained by memory. */
> > if (writecnt>  65536 || readcnt>  65536)
> > return SPI_INVALID_LENGTH;
> > if (bufsize != oldbufsize) {
> > bufsize = max(writecnt + readcnt, 260);
> > /* Never shrink. realloc() calls are expensive. */
> > if (bufsize>  oldbufsize) {
> > bufout = realloc(bufout, bufsize);
> > if (!bufout) {
> > 	fprintf(stderr, "Out of memory!\n");
> >
> > @@ -109,6 +111,7 @@
> >    			free(bufout);
> >    		exit(1);
> >    	}
> > oldbufsize = bufsize;
> > }
> >
> > memcpy(bufout, writearr, writecnt);
> > Index: flashrom-realloc_overhead/ft2232_spi.c
> >
> > --- flashrom-realloc_overhead/ft2232_spi.c	(Revision 777)
> >  +++ flashrom-realloc_overhead/ft2232_spi.c	(Arbeitskopie)
> >  @@ -200,14 +200,22 @@
> >    static unsigned char *buf = NULL;
> >    /* failed is special. We use bitwise ops, but it is essentially bool. */
> >    int i = 0, ret = 0, failed = 0;
> >
> > int bufsize;
> >
> > static int oldbufsize = 0;
> >
> > if (writecnt>  65536 || readcnt>  65536)
> > return SPI_INVALID_LENGTH;
> > buf = realloc(buf, writecnt + readcnt + 100);
> > if (!buf) {
> > fprintf(stderr, "Out of memory!\n");
> > exit(1); // -1
> >
> > /* buf is not used for the response from the chip. */
> >
> > bufsize = max(writecnt + 9, 260);
> >
> > /* Never shrink. realloc() calls are expensive. */
> >
> > if (bufsize>  oldbufsize) {
> > buf = realloc(buf, bufsize);
> > if (!buf) {
> > 	fprintf(stderr, "Out of memory!\n");
> > 	exit(1);
> > }
> > oldbufsize = bufsize;
> >
> > }
> >
> > /*
>
> =---------------------
>   paul fox, [e-mail] (arlington, ma, where it's 47.1 degrees)
>
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I always assume any memory resizing calls are expensive due to the 
content checking; if shrinking check that the space isn't used, if 
expanding check that the adjacent memory segments aren't used, or 
searching for a space that isn't used.

The patch looks fine and I see no problems so:
Acked-by: Sean Nelson