On Thu, May 03, 2012 at 10:48:53AM +0200, Sasha Levin wrote: > On Thu, May 3, 2012 at 10:44 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > 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@xxxxxxxxxx> 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@xxxxxxxxxx> 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. > > I did intend to allow him to change the value while the device is > running, if we don't want to allow that then it's easy. > > >> > 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. > > I thought you meant that we apply the patch with threshold set at > 0/disabled, and based on future tests we will enable it for specific > devices and set best values for threshold, no? Exactly the opposite. I meant each driver sets the value and we test it to find a good value. Drivers that don't do anything use existing kmalloc code. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
