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Subject: Re: [virtio-dev] RFC: Doorbell suppression, packed-ring mode and hardware offload

On 2019/2/14 äå12:04, Michael S. Tsirkin wrote:
On Thu, Feb 14, 2019 at 11:34:22AM +0800, Jason Wang wrote:
On 2019/2/14 äå1:30, Michael S. Tsirkin wrote:
On Wed, Feb 13, 2019 at 06:33:50PM +0800, Jason Wang wrote:
On 2019/2/13 äå1:35, Michael S. Tsirkin wrote:
On Tue, Feb 12, 2019 at 04:47:08PM +0000, David Riddoch wrote:
I would prefer to have the write barrier before writing the idx.
Well that's driver overhead for something device might never utilise in
a given workload. If we are optimizing let's optimize for speed.
I think doing the barrier before writing idx is best for speed (see below).
I don't see it below:(
Sorry, I'm not being clear. If the write barrier is before the idx, then a
PV device can read the idx, do a single rmb and read a whole bunch of
descriptors. As things stand today a PV device has to do an rmb for each
descriptor that it reads.

I'm not sure, but this may be what Jason meant when he said "prefetch".

Oh. Right. So for PV it's easy to convert an rmb to a data dependency
instead.  It remains to be seen whether an extra cache miss per
batch is cheaper or more expensive than such a dependency
per descriptor.

I hope Jason can experiment and let us know.

To clarify, actually I did play someting like:

if (desc_is_avail((last_avail + 32) % size))

  ÂÂÂ [last_avail, last_avail + 32] is avail [1]

else if (desc_is_avail(last_avail + 1) % size))

  ÂÂÂ last_avail is avail


  ÂÂ no new

And looks like [1] depends on the driver to do:

for all descriptors

  ÂÂÂ update desc (but not make it available for the flag)


for all descriptors

  ÂÂÂ update desc.flag

This means if last_avail + 32 is avail, no need to check flags of
[last_avail, last_avail + 31] since the descriptor content (except for the
flag) is guaranteed to be correct.
We can try looking into that, sure. It's not just wmb though which is
anyway free on x86. There are cases of e.g. chained descriptors
to consider where we do not want device to see a partial chain.

If we saw the last descriptor is in a chain, it's still doesn't harm, just
read more?
So it's possible to add a variant that sticks a wmb after each flag
update for sure. However if you are writing out batches that's an
upfront cost and what you gain is prefetch on the other side which is


I get ~10% PPS improvement.

I suspect most of the gain isn't in the check at all though.

The gain was:

1) batching reading descriptors, saving the times of userspace access
Yes that needs to be fixed. The way linux vhost driver does userspace
accesses right now with stac/clac and memory barriers within a loop is
plain silly.  Designing a host/guest interface to work around that kind
of inefficiency makes no sense, it's purely an implementation issue.

Actually, it has other advantages, consider the case when driver is faster, batch reading may reduce the cache contention on the descriptor ring (when the ring is almost full).

2) reduce the rmb(), otherwise you need rmb per descriptor
Well not really, you can
- validate a batch then do a single rmb
- replace rmb with a dependency

It's probably reading descriptors and maybe loop unrolling.

I'm not quite understand why unrolling help in this case.
The amount of operations in this loop is tiny but not tiny
enough for gcc to decide it's free. Take a look

#include <stdio.h>

struct desc {
         unsigned long long a;
         unsigned len;
         unsigned short type;
         unsigned short flags;
#ifdef UNROLL
int foo(struct desc *desc)
      unsigned short aflags = 0xffff;
      unsigned short oflags = 0x0;

      int i;

         for (i=0; i < 4; ++i) {
              aflags &= desc[i].flags;
              oflags |= desc[i].flags;

         return aflags == oflags;

$ gcc -Wall -DUNROLL -O2 -c l.c

I think you mean without -DUNROLL.

0000000000000000 <foo>:
    0:   48 8d 47 0e             lea    0xe(%rdi),%rax
    4:   31 c9                   xor    %ecx,%ecx
    6:   48 83 c7 4e             add    $0x4e,%rdi
    a:   be ff ff ff ff          mov    $0xffffffff,%esi
    f:   0f b7 10                movzwl (%rax),%edx
   12:   48 83 c0 10             add    $0x10,%rax
   16:   21 d6                   and    %edx,%esi
   18:   09 d1                   or     %edx,%ecx
   1a:   48 39 f8                cmp    %rdi,%rax
   1d:   75 f0                   jne    f <foo+0xf>
   1f:   31 c0                   xor    %eax,%eax
   21:   66 39 ce                cmp    %cx,%si
   24:   0f 94 c0                sete   %al
   27:   c3                      retq

$ gcc -Wall -DUNROLL -O2 -c l.c

Disassembly of section .text:

0000000000000000 <foo>:
    0:   0f b7 47 1e             movzwl 0x1e(%rdi),%eax
    4:   0f b7 77 2e             movzwl 0x2e(%rdi),%esi
    8:   0f b7 4f 0e             movzwl 0xe(%rdi),%ecx
    c:   89 c2                   mov    %eax,%edx
    e:   09 f0                   or     %esi,%eax
   10:   21 f2                   and    %esi,%edx
   12:   09 c8                   or     %ecx,%eax
   14:   21 ca                   and    %ecx,%edx
   16:   0f b7 4f 3e             movzwl 0x3e(%rdi),%ecx
   1a:   09 c8                   or     %ecx,%eax
   1c:   21 ca                   and    %ecx,%edx
   1e:   66 39 c2                cmp    %ax,%dx
   21:   0f 94 c0                sete   %al
   24:   0f b6 c0                movzbl %al,%eax
   27:   c3                      retq

See? gcc just does not unroll agressively enough.

Ok, so this is just for avoiding branches.

Something like:

bool fast_get_descriptors(vq, desc)

	copy X descriptors
	u16 aflags = 0xffff;
	u16 oflags = 0x0;
	for (i = 0; i < X; ++i) {
		aflags &= desc[i].flags;
		oflags |= desc[i].flags;
	/* all flags must be same */
	if (aflags != oflags)
		return false;

	return is_avail(vq->wrapcount, aflags);

This may work. It should be a little bit less efficient but without any
assumption of driver if I understand correctly.

I also think 32 is very aggressive, I would start with 8.

Maybe, or adding some heuristic to predict it.

The reason I say 8 is because it's two cache lines which
is typically what cpu pulls in anyway (1+linear pfetch).
Anything more might have more cost as it might
put more pressure on the cache. Further
static sized short loops can be unrolled by compiler if
you ask it nicely, which saves a bunch of branching,
speculation etc. Dynamic sized ones can't,
you end up with extra math and branches in the loop.

You're probably right here, but need some benchmark to check.


With above, you could try:

	if (fast_get_descriptors(vq, desc))
	 ÂÂÂ [last_avail, last_avail + 32] is avail [1]
	else if (desc_is_avail(last_avail + 1) % size))
	 ÂÂÂ last_avail is avail
   	 	no new

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