On Wed, 2011-07-20 at 16:41 -0300, Marcelo Tosatti wrote:
> On Wed, Jul 20, 2011 at 03:11:58PM +0300, Sasha Levin wrote:
> > Currently the method of dealing with an IO operation on a bus (PIO/MMIO)
> > is to call the read or write callback for each device registered
> > on the bus until we find a device which handles it.
> >
> > Since the number of devices on a bus can be significant due to ioeventfds
> > and coalesced MMIO zones, this leads to a lot of overhead on each IO
> > operation.
> >
> > Instead of registering devices, we now register ranges which points to
> > a device. Lookup is done using an efficient bsearch instead of a linear
> > search.
> >
> > This should speed up all IO operations generated by the guest.
>
> Some numbers, please.
>
I'm not sure how to measure performance in this case. You'd need to
measure access times to each device before and after the patch, and
average them.
It replaces a linear lookup with a binary one, it's not an absolute
improvement.
> > +int kvm_io_bus_find_closest_dev_idx(struct kvm_io_bus *bus,
> > + gpa_t addr, int len)
> > +{
> > + int start = 0, end = bus->dev_count - 1;
> > +
> > + if (bus->dev_count == 0)
> > + return -1;
> > +
> > + while (start <= end) {
> > + int mid = (start + end) / 2;
> > + struct kvm_io_range *range = &bus->range[mid];
> > +
> > + if (addr > range->addr)
> > + start = mid + 1;
> > + else if (addr < range->addr)
> > + end = mid - 1;
>
> If mid is zero, this assigns end = -1?
>
Yes. Next step would be to exit the while ().
--
Sasha.
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