On 2022-07-01 12:33, John Garry wrote:
On 01/07/2022 04:56, Feng Tang wrote:

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

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.

This idea is interesting, but it would mean a bit more fiddly work to keep things in sync when magazines are allocated, freed and swapped around. It seems like the kind of non-obvious thing that might make sense if it gave a significant improvement in cache locality or something like that, but for simply fixing an allocation size it feels a bit too wacky.

From my perspective, indeed I'd rather do the simple thing for now to address the memory wastage issue directly, then we can do the deeper performance analysis on top to see if further tweaking of magazine sizes and/or design is justified.

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