Re: using cache for virtio allocations?

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 03, 2012 at 10:38:56AM +0200, Sasha Levin wrote:
> On Thu, May 3, 2012 at 9:32 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Thu, May 03, 2012 at 07:51:18AM +0200, Sasha Levin wrote:
> >> On Thu, May 3, 2012 at 7:29 AM, Michael S. Tsirkin <mst@redhat.com> wrote:
> >> > Sasha, didn't you have a patch to allocate
> >> > things using cache in virtio core?
> >> > What happened to it?
> >> >
> >> > Thanks,
> >> > MST
> >>
> >> It got stuck due to several things, and I got sidetracked, sorry. Here
> >> are the outstanding issues:
> >>
> >> 1. Since now we can allocate a descriptor either using kmalloc or from
> >> the cache, we need a new flag in vring_desc to know how to free it, it
> >> seems a bit too intrusive,
> >> and I couldn't thing of a better
> >> alternative.
> >
> > Since that is guest visible it does not sound great, I agree.
> >
> > Three ideas:
> > 1. The logic looks at descriptor size so can we just read
> >   desc.len before free and rerun the same math?
> 
> It'll break every time the value is changed (either by the user or by
> some dynamic algorithm thingie).

Yes but did you intend to implement such complex logic?
If not let's not over-engineer.

> > 2. For -net the requests are up to max_skb_frags + 2 in size, right?
> >   Does it make sense to just use cache for net, always?
> >   That would mean a per device flag.
> 
> Yup, it could work.
> 
> > 3. Allocate a bit more and stick extra data before the 1st descriptor.
> 
> I guess it'll work, but it just seems a bit ugly :)

An understatement.

> >> 2. Rusty has pointed out that no one is going to modify the default
> >> value we set, and we don't really have a good default value to put
> >> there (at least, we haven't agreed on a specific value). Also, you
> >> have noted that it should be a per-device value, which complicates
> >> this question further since we probably want a different value for
> >> each device type.
> >>
> >> While the first one can be solved easily with a blessing from the
> >> maintainers, the second one will require testing on various platforms,
> >> configurations and devices to select either the best "magic" value, or
> >> the best algorithm to play with threshold.
> >
> > Not sure about platforms but for devices that's right.
> > But this really only means we only change what we tested.
> > eg see what is good for net and change net in a way
> > that others will keep using old code.
> 
> It'll work only if there will be someone following up and actually
> testing it, since regular users won't be testing it at all (with it
> being defaulted to off and everything).

Not sure I understand. Whatever patch gets applied will be
tested beforehand.

-- 
MST