On Wed, 18 May 2016 11:21:59 +0300 "Michael S. Tsirkin" <m...@redhat.com> wrote:
> On Wed, May 18, 2016 at 10:16:31AM +0200, Jesper Dangaard Brouer wrote: > > > > On Tue, 17 May 2016 09:38:37 +0800 Jason Wang <jasow...@redhat.com> wrote: > > > > > >> And if tx_queue_length is not power of 2, > > > >> we probably need modulus to calculate the capacity. > > > > Is that really that important for speed? > > > > > > Not sure, I can test. > > > > In my experience, yes, adding a modulus does affect performance. > > How about simple > if (unlikely(++idx > size)) > idx = 0; So, you are exchanging an AND-operation with a mask, for a branch-operation. If the branch predictor is good enough in the CPU and code-"size" use-case, then I could be just as fast. I've actually played with a lot of different approaches: https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/include/linux/alf_queue_helpers.h I cannot remember the exact results. I do remember micro benchmarking showed good results with the advanced "unroll" approach, but IPv4 forwarding, where I know I-cache is getting evicted, showed best results with the more simpler implementations. > > > > > > Right, this sounds a good solution. > > > > Good idea. > > I'm not that sure - it's clearly wasting memory. Rounding up to power of two. In this case I don't think the memory wast is too high. As we are talking about max 16 bytes elements. I am concerned about memory in another way. We need to keep these arrays/rings small, due to data cache usage. A 4096 ring queue is bad because e.g. 16*4096=65536 bytes, and typical L1 cache is 32K-64K. As this is a circular buffer, we walk over this memory all the time, thus evicting the L1 cache. -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer
