Hi Andrew,
On 17/12/2021 15:56, Andrew Cooper wrote:
On 16/11/2021 00:41, Andrew Cooper wrote:
On 16/11/2021 00:36, Stefano Stabellini wrote:
On Thu, 11 Nov 2021, Julien Grall wrote:
On 11/11/2021 17:57, Andrew Cooper wrote:
There are exactly 3 callers of sort() in the hypervisor.
Both arm callers pass in NULL for the swap function. While this might seem
like an attractive option at first, it causes generic_swap() to be used
which
forced a byte-wise copy. Provide real swap functions which the compiler can
optimise sensibly.
I understand the theory, but none of the two calls are in hot paths or deal
with large set on Arm. So I am rather hesitant to introduce specialised swap
in each case as it doesn't seem we will gain much from this change.
While I like Andrew's optimization, like Julien, I would also rather not
have to introduce specialized swap functions any time a sort() is
called. Why not keeping around generic_swap as extern gnu_inline in
sort.h as well so that the ARM callers can simply:
sort(bootinfo.mem.bank, bootinfo.mem.nr_banks, sizeof(struct membank),
cmp_memory_node, generic_swap);
?
That looks like a good option, although it would still result in a
small increase in bloat.
That is basically what Jan suggested.
I'm still unconvinced that you actually want to be doing byte-wise
swapping, even if it isn't a hotpath. A custom swap function will
compile to less code than using generic_swap() like this, and run faster.
ARM Ping.
I really think this is a bad course of action. If you're going to
insist on it, I want something in writing.
I agree with all the points you wrote. However, you also have to put
into perspective how this is used. On Arm, the two callers happen either
during boot a domain creation. Neither of them have a significant impact
on the time spent. In fact, I would be surprised if you notice the change.
So unless there are other (good) reasons to suppress generic_swap(), I
still want to be able to use generic_swap() when the performance doesn't
matter (like the two Arm callers).
Cheers,
--
Julien Grall
