Re: [PATCH 1/2] kfifo: round up the fifo size power of 2

From: Stefani Seibold
Date: Wed Oct 31 2012 - 16:31:41 EST


Am Dienstag, den 30.10.2012, 23:52 -0700 schrieb Andrew Morton:
> On Wed, 31 Oct 2012 07:30:33 +0100 Stefani Seibold <stefani@xxxxxxxxxxx> wrote:
>
> > > Yes, and I guess the same to give them a 64-element one.
> > >
> > > >
> > > > If there's absolutely no prospect that the kfifo code will ever support
> > > > 100-byte fifos then I guess we should rework the API so that the caller
> > > > has to pass in log2 of the size, not the size itself. That way there
> > > > will be no surprises and no mistakes.
> > > >
> > > > That being said, the power-of-2 limitation isn't at all intrinsic to a
> > > > fifo, so we shouldn't do this. Ideally, we'd change the kfifo
> > > > implementation so it does what the caller asked it to do!
> > >
> > > I'm fine with removing the power-of-2 limitation. Stefani, what's your
> > > comment on that?
> > >
> >
> > You can't remove the power-of-2-limitation, since this would result in a
> > performance decrease (bit wise and vs. modulo operation).
>
> Probably an insignificant change in performance.
>
> It could be made much smaller by just never doing the modulus operation
> - instead do
>
> if (++index == max)
> index = 0;
>
> this does introduce one problem: it's no longer possible to distinguish
> the "full" and "empty" states by comparing the head and tail indices.
> But that is soluble.
>

And you will increase the code size, since kfifo_put and kfifo_get are
inline code. Also the speculative execution path of modern CPUs must
kick away the pipeline in case of are false branch prediction.

> > Andrew is right, this is an API miss design. So it would be good to
> > rework the kfifo_init () and kfifo_alloc() to pass in log2 of the size,
> > not the size itself.
>
> The power-of-2 thing is just a restriction in the current
> implementation - it's not a good idea to cement that into the
> interface. Of course, it could later be uncemented if the
> implementation's restriction was later relaxed.

The power-of-2 thing is a design restriction, a balance between
performance and code size and usability.


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