Re: [RFC v1 04/14] bus1: util - fixed list utility library

[Arnd Bergmann]

On Thursday, October 27, 2016 2:48:46 PM CEST David Herrmann wrote:
> On Thu, Oct 27, 2016 at 2:37 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Wed, Oct 26, 2016 at 09:18:00PM +0200, David Herrmann wrote:
> >> +             e = kmalloc_array(sizeof(*e), BUS1_FLIST_BATCH + 1, gfp);
> >
> >> +#define BUS1_FLIST_BATCH (1024)
> >
> >> +struct bus1_flist {
> >> +     union {
> >> +             struct bus1_flist *next;
> >> +             void *ptr;
> >> +     };
> >> +};
> >
> > So that's an allocation of 8*(1024+1), or slightly more than 2 pages.
> >
> > kmalloc will round up to the next power of two, so you'll end up with an
> > allocation of 16*1024, wasting a whopping 8184 bytes per such allocation
> > in slack space.
> >
> > Please consider using 1023 or something for your batch size, 511 would
> > get you to exactly 1 page which would be even better.
> 
> Thanks for the hint! 511 looks like the obvious choice. Maybe even
> (PAGE_SIZE / sizeof(long) - 1). I will put a comment next to the
> definition.
> 
> 

PAGE_SIZE can be up to 64KB though, so that might lead wasting a lot
of memory.

	Arnd