Re: [Patch v3 1/3] iommu: avoid unnecessary magazine allocations

[Cong Wang <xiyou.wangcong@gmail.com>]

On Tue, Jan 21, 2020 at 3:11 AM Robin Murphy <robin.murphy@arm.com> wrote:
>
> On 18/12/2019 4:39 am, Cong Wang wrote:
> > The IOVA cache algorithm implemented in IOMMU code does not
> > exactly match the original algorithm described in the paper
> > "Magazines and Vmem: Extending the Slab Allocator to Many
> > CPUs and Arbitrary Resources".
> >
> > Particularly, it doesn't need to free the loaded empty magazine
> > when trying to put it back to global depot. To make it work, we
> > have to pre-allocate magazines in the depot and only recycle them
> > when all of them are full.
> >
> > Before this patch, rcache->depot[] contains either full or
> > freed entries, after this patch, it contains either full or
> > empty (but allocated) entries.
>
> How much additional memory overhead does this impose (particularly on
> systems that may have many domains mostly used for large, long-term
> mappings)? I'm wary that trying to micro-optimise for the "churn network
> packets as fast as possible" case may penalise every other case,
> potentially quite badly. Lower-end embedded systems are using IOMMUs in
> front of their GPUs, video codecs, etc. precisely because they *don't*
> have much memory to spare (and thus need to scrape together large
> buffers out of whatever pages they can find).

The calculation is not complicated: 32 * 6 * 129 * 8 = 198144 bytes,
which is roughly 192K, per domain.

>
> But on the other hand, if we were to go down this route, then why is
> there any dynamic allocation/freeing left at all? Once both the depot
> and the rcaches are preallocated, then AFAICS it would make more sense
> to rework the overflow case in __iova_rcache_insert() to just free the
> IOVAs and swap the empty mag around rather than destroying and
> recreating it entirely.

It's due to the algorithm requires a swap(), which can't be done with
statically allocated magzine. I had the same thought initially but gave it
up quickly when realized this.

If you are suggesting to change the algorithm, it is not a goal of this
patchset. I do have plan to search for a better algorithm as the IOMMU
performance still sucks (comparing to no IOMMU) after this patchset,
but once again, I do not want to change it in this patchset.

(My ultimate goal is to find a spinlock-free algorithm, otherwise there is
no way to make it close to no-IOMMU performance.)

>
> Perhaps there's a reasonable compromise wherein we don't preallocate,
> but still 'free' empty magazines back to the depot, such that busy
> domains will quickly reach a steady-state. In fact, having now dug up
> the paper at this point of writing this reply, that appears to be what
> fig. 3.1b describes anyway - I don't see any mention of preallocating
> the depot.

That paper missed a lot of things, it doesn't even recommend a size
of a depot or percpu cache. For implementation, we still have to
think about those details, including whether to preallocate memory.

Thanks.