On Wed, May 18, 2016 at 06:42:10PM +0800, Jason Wang wrote: > > > On 2016年05月18日 17:55, Michael S. Tsirkin wrote: > >On Wed, May 18, 2016 at 11:21:29AM +0200, Jesper Dangaard Brouer wrote: > >>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. > >This is all assuming you can somehow batch operations. > >We can do this for transmit sometimes (when linux > >is the source of the packets) but not always. > > > >>>>>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. > >It almost doubles it. > >E.g. queue size of 10000 (rather common) will become 16K, wasting 6K. > > It depends on the user, e.g default tx_queue_len is around 1000 for real > cards. If we really care about the wasting, we can add a threshold and fall > back to normal linked list during resizing.
That looks like a lot of complexity. > > > >>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. > >Depends on the usage I guess. > >Entries pointed to are much bigger, and you are > >going to access them - is this really an issue? > >If yes this shouldn't be that hard to fix ... > > > >>-- > >>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
