On Sun, Dec 04, 2011 at 01:06:37PM +0200, Michael S. Tsirkin wrote: > On Sat, Dec 03, 2011 at 01:50:28PM +0200, Sasha Levin wrote: > > On Fri, 2011-12-02 at 11:16 +1030, Rusty Russell wrote: > > > On Thu, 1 Dec 2011 12:26:42 +0200, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > > > > On Thu, Dec 01, 2011 at 10:09:37AM +0200, Sasha Levin wrote: > > > > > On Thu, 2011-12-01 at 09:58 +0200, Michael S. Tsirkin wrote: > > > > > > We'll presumably need some logic to increment is back, > > > > > > to account for random workload changes. > > > > > > Something like slow start? > > > > > > > > > > We can increment it each time the queue was less than 10% full, it > > > > > should act like slow start, no? > > > > > > > > No, we really shouldn't get an empty ring as long as things behave > > > > well. What I meant is something like: > > > > > > I was thinking of the network output case, but you're right. We need to > > > distinguish between usually full (eg. virtio-net input) and usually > > > empty (eg. virtio-net output). > > > > > > The signal for "we to pack more into the ring" is different. We could > > > use some hacky heuristic like "out == 0" but I'd rather make it explicit > > > when we set up the virtqueue. > > > > > > Our other alternative, moving the logic to the driver, is worse. > > > > > > As to fading the effect over time, that's harder. We have to deplete > > > the ring quite a few times before it turns into always-indirect. We > > > could back off every time the ring is totally idle, but that may hurt > > > bursty traffic. Let's try simple first? > > > > I tried to take a different approach, and tried putting the indirect > > descriptors in a kmem_cache as Michael suggested. The benchmarks showed > > that this way virtio-net actually worked faster with indirect on even in > > a single stream. > > > > Maybe we can do that instead of playing with threshold for now. > > > > The question here, how much wasted space we can afford? since indirect > > descriptors would have to be the same size we'd have a bunch of them > > wasted in the cache. Ofcourse we can make that configurable, but how > > much is ok by default? > > I think it's a good idea to make that per-device. > For network at least, each skb already has overhead of > around 1/2 K, so using up to 1/2K more seems acceptable. > But even if we went up to MAX_SKB_FRAGS+2, it would be > only 1K per ring entry, I got this wrong - descriptor is 16 bytes, so MAX_SKB_FRAGS+2 descriptors would be less than 300 bytes overhead per packet. That's not a lot. > so for a ring of 256 entries, we end up with > 256K max waste. That's not that terrible. > > But I'd say let's do some benchmarking to figure out > the point where the gains are becoming very small. > > -- > > > > Sasha. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
