Cornelia Huck <cornelia.huck@xxxxxxxxxx> writes: > On Fri, 13 Feb 2015 13:22:09 +1030 > Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote: > >> "Michael S. Tsirkin" <mst@xxxxxxxxxx> writes: >> > On Tue, Feb 10, 2015 at 12:02:37PM +1030, Rusty Russell wrote: >> >> Jason Wang <jasowang@xxxxxxxxxx> writes: >> >> > This patch enables the interrupt coalescing setting through ethtool. >> >> >> >> The problem is that there's nothing network specific about interrupt >> >> coalescing. I can see other devices wanting exactly the same thing, >> >> which means we'd deprecate this in the next virtio standard. >> >> >> >> I think the right answer is to extend like we did with >> >> vring_used_event(), eg: >> >> >> >> 1) Add a new feature VIRTIO_F_RING_COALESCE. >> >> 2) Add another a 32-bit field after vring_used_event(), eg: >> >> #define vring_used_delay(vr) (*(u32 *)((vr)->avail->ring[(vr)->num + 2])) >> >> >> >> This loses the ability to coalesce by number of frames, but we can still >> >> do number of sg entries, as we do now with used_event, and we could >> >> change virtqueue_enable_cb_delayed() to take a precise number if we >> >> wanted. >> > >> > But do we expect delay to be update dynamically? >> > If not, why not stick it in config space? >> >> Hmm, we could update it dynamically (and will, in the case of ethtool). >> But it won't be common, so we could append a field to >> virtio_pci_common_cfg for PCI. >> >> I think MMIO and CCW would be easy to extend too, but CC'd to check. > > If this is a simple extension of the config space, it should just work > for ccw (the Linux guest driver currently uses 0x100 as max config > space size, which I grabbed from pci at the time I wrote it). > > But looking at this virtio_pci_common_cfg stuff, it seems to contain a > lot of things that are handled via ccws on virtio-ccw (like number of > queues or device status). Having an extra ccw just for changing this > delay value seems like overkill. Yes, possibly. > On the basic topic of interrupt coalescing: With adapter interrupts, > virtio-ccw already has some kind of coalescing: The summary indicator > is set just once and an interrupt is made pending, then individual > queue indicators are switched on and no further interrupt is generated > if the summary indicator has not been cleared by the guest yet. I'm not > sure how it would be different if an individual queue indicator is > switched on later. Chances are that the guest code processing the > indicators has not even yet processed to that individual indicator, so > it wouldn't matter if it was set delayed. It is probably something that > has to be tried out. The network driver will do this at the virtio level too: no more rx interrupts will be received until all packets have been processed. But it is particularly useful for network transmit interrupts: we want to be notified of the packet's finishing, but a little delay (hence more batching) is better. For rx, I can envision a case where the guest is too fast and thus keeps getting interrupted after each packet. A user might decide to trade off some latency to increase batching; seems like a bit like a benchmark hack to me, though... Cheers, Rusty. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
