Messages in this thread | | | Subject | Re: [PATCH 1/2] kfifo: round up the fifo size power of 2 | From | Stefani Seibold <> | Date | Wed, 31 Oct 2012 21:31:03 +0100 |
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Am Dienstag, den 30.10.2012, 23:52 -0700 schrieb Andrew Morton: > On Wed, 31 Oct 2012 07:30:33 +0100 Stefani Seibold <stefani@seibold.net> 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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