Messages in this thread | | | Subject | Re: [PATCH net-next] tuntap: introduce tx skb ring | From | Jason Wang <> | Date | Wed, 18 May 2016 18:42:10 +0800 |
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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" <mst@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 <jasowang@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.
> >> 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
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