On 2018/12/26 下午11:06, Michael S. Tsirkin wrote:
On Wed, Dec 26, 2018 at 12:03:50PM +0800, Jason Wang wrote:
On 2018/12/26 上午12:41, Michael S. Tsirkin wrote:
Hi!
I was just wondering: packed ring batches things naturally.
E.g.
user_access_begin
check descriptor valid
smp_rmb
copy descriptor
user_access_end
But without speculation on the descriptor (which may only work for in-order
or even a violation of spec). Only one two access of a single descriptor
could be batched. For split ring, we can batch more since we know how many
descriptors is pending. (avail_idx - last_avail_idx).
Anything I miss?
Thanks
just check more descriptors in a loop:
user_access_begin
for (i = 0; i < 16; ++i) {
if (!descriptor valid)
break;
smp_rmb
copy descriptor
}
user_access_end
you don't really need to know how many there are
ahead of the time as you still copy them 1 by one.
So let's see the case of split ring
user_access_begin
n = avail_idx - last_avail_idx (1)
n = MIN(n, 16)
smp_rmb
read n entries from avail_ring (2)
for (i =0; i <n; i++)
copy descriptor (3)
user_access_end
Consider for the case of heavy workload. So for packed ring, we have 32
times of userspace access and 16 times of smp_rmb()
For split ring we have
(1) 1 time
(2) 2 times at most
(3) 16 times
19 times of userspace access and 1 times of smp_rmb(). In fact 2 could
be eliminated with in order. 3 could be batched completely with in order
and partially when out of order.
I don't see how packed ring help here especially consider lfence on x86
is more than memory fence, it prevents speculation in fact.
Thanks
So packed layout should show the gain with this approach.
That could be motivation enough to finally enable vhost packed ring
support.
Thoughts?
