Re: [Xen-devel] "tcp: refine TSO autosizing" causes performance regression on Xen

From: Rick Jones
Date: Wed Apr 15 2015 - 16:09:24 EST

On 04/15/2015 11:32 AM, Eric Dumazet wrote:
On Wed, 2015-04-15 at 11:19 -0700, Rick Jones wrote:

Well, I'm not sure that it is George and Jonathan themselves who don't
want to change a sysctl, but the customers who would have to tweak that
in their VMs?

Keep in mind some VM users install custom qdisc, or even custom TCP

That could very well be, though I confess I've not seen that happening in my little corner of the cloud. They tend to want to launch the VM and go. Some of the more advanced/sophisticated ones might tweak a few things but my (admittedly limited) experience has been they are few in number. They just expect it to work "out of the box" (to the extent one can use that phrase still).

It's kind of ironic - go back to the (early) 1990s when NICs generated a completion interrupt for every individual tx completion (and incoming packet) and all everyone wanted to do was coalesce/avoid interrupts. I guess that has gone rather far. And today to fight bufferbloat TCP gets tweaked to favor quick tx completions. Call it cycles, or pendulums or whatever I guess.

I wonder just how consistent tx completion timings are for a VM so a virtio_net or whatnot in the VM can pick a per-device setting to advertise to TCP? Hopefully, full NIC emulation is no longer a thing and VMs "universally" use a virtual NIC interface. At least in my little corner of the cloud, emulated NICs are gone, and good riddance.

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