Re: [PATCH] iommu/iova: change IOVA_MAG_SIZE to 127 to save memory

[John Garry]

On 01/07/2022 04:56, Feng Tang wrote:
> > > inclination.
> > ok, what you are saying sounds reasonable. I just remember that when we
> > analyzed the longterm aging issue that we concluded that the FQ size and its
> > relation to the magazine size was a factor and this change makes me a little
> > worried about new issues. Better the devil you know and all that...
> >
> > Anyway, if I get some time I might do some testing to see if this change has
> > any influence.
> >
> > Another thought is if we need even store the size in the iova_magazine? mags
> > in the depot are always full. As such, we only need worry about mags loaded
> > in the cpu rcache and their sizes, so maybe we could have something like
> > this:
> >
> > struct iova_magazine {
> > -       unsigned long size;
> >         unsigned long pfns[IOVA_MAG_SIZE];
> > };
> >
> > @@ -631,6 +630,8 @@ struct iova_cpu_rcache {
> >         spinlock_t lock;
> >         struct iova_magazine *loaded;
> >         struct iova_magazine *prev;
> > +       int loaded_size;
> > +       int prev_size;
> > };
> >
> > I haven't tried to implement it though..
> I have very few knowledge of iova, so you can chose what's the better
> solution. I just wanted to raise the problem and will be happy to see
> it solved:)

I quickly tested your patch for performance and saw no noticeable 
difference, which is no surprise.

But I'll defer to Robin if he thinks that your patch is a better 
solution - I would guess that he does. For me personally I would prefer 
that this value was not changed, as I mentioned before.

thanks,
John