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lguest: avoid sending interrupts to Guest when no activity occurs.

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If we track how many buffers we've used, we can tell whether we really
need to interrupt the Guest.  This happens as a side effect of
spurious notifications.

Spurious notifications happen because it can take a while before the
Host thread wakes up and sets the VRING_USED_F_NO_NOTIFY flag, and
meanwhile the Guest can more notifications.

A real fix would be to use wake counts, rather than a suppression
flag, but the practical difference is generally in the noise: the
interrupt is usually coalesced into a pending one anyway so we just
save a system call which isn't clearly measurable.

				Secs	Spurious IRQS
1G TCP Guest->Host:		3.93	58
1M normal pings:		100	72
1M 1k pings (-l 120):		57	492904

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell 2009-06-12 22:27:11 -06:00
parent 38bc2b8c56
commit 95c517c09b
1 changed files with 9 additions and 0 deletions
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9
Documentation/lguest/lguest.c
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@ -151,6 +151,9 @@ struct virtqueue
/* Last available index we saw. */
u16 last_avail_idx;
/* How many are used since we sent last irq? */
unsigned int pending_used;
/* Eventfd where Guest notifications arrive. */
int eventfd;
@ -556,6 +559,11 @@ static void trigger_irq(struct virtqueue *vq)
{
unsigned long buf[] = { LHREQ_IRQ, vq->config.irq };
/* Don't inform them if nothing used. */
if (!vq->pending_used)
return;
vq->pending_used = 0;
/* If they don't want an interrupt, don't send one, unless empty. */
if ((vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)
&& lg_last_avail(vq) != vq->vring.avail->idx)
@ -647,6 +655,7 @@ static void add_used(struct virtqueue *vq, unsigned int head, int len)
/* Make sure buffer is written before we update index. */
wmb();
vq->vring.used->idx++;
vq->pending_used++;
}
/* And here's the combo meal deal. Supersize me! */