netdev - Re: [PATCH net-next 3/3] net: auto-tune mergeable rx buffer size for improved performance

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Date:	Thu, 26 Dec 2013 12:06:23 -0800
From:	Michael Dalton <mwdalton@...gle.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	lf-virt <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH net-next 3/3] net: auto-tune mergeable rx buffer size for
 improved performance

On Mon, Dec 23, 2013 at 4:51 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> OK so a high level benchmark shows it's worth it,
> but how well does the logic work?
> I think we should make the buffer size accessible in sysfs
> or debugfs, and look at it, otherwise we don't really know.
>
Exporting the size sounds good to me, it is definitely an
important metric and would give more visibility to the admin.

Do you have a preference for implementation strategy? I was
thinking just add a DEVICE_ATTR to create a read-only sysfs file,
'mergeable_rx_buffer_size', and return a space-separated list of the
current buffer size (computed from the average packet size) for each
receive queue. -EINVAL or a similar error could be returned if the
netdev was not configured for mergeable rx buffers.

> I don't get the real motivation for this.
>
> We have skbs A,B,C sharing a page, with chunk D being unused.
> This randomly charges chunk D to an skb that ended up last
> in the page.
> Correct?
> Why does this make sense?

The intent of this code is to adjust the SKB true size for
the packet. We should completely use each packet buffer except
for the last buffer. For all buffers except the last buffer, it
should be the case that 'len' (bytes received) = buffer size. For
the last buffer, this code adjusts the truesize by comparing the
approximated buffer size with the bytes received into the buffer,
and adding the difference to the SKB truesize if the buffer size
is greater than the number of bytes received.

We approximate the buffer size by using the last packet buffer size
from that same page, which as you have correctly noted may be a buffer
that belongs to a different packet on the same virtio-net device. This
buffer size should be very close to the actual buffer size because our
EWMA estimator uses a high weight (so the packet buffer size changes very
slowly) and there are only a handful packets on a page (even order-3).

> Why head_skb only? Why not full buffer size that comes from host?
> This is simply len.

Sorry, I believe this code fragment should be clearer. Basically, we
have a corner case in that for packets with size <= GOOD_COPY_LEN, there
are no frags because page_to_skb() already unref'd the page and the entire
packet contents are copied to skb->data. In this case, the SKB truesize
is already accurate and should not be updated (and it would be unsafe to
access page->private as page is already unref'd).

I'll look at the above code again and cleanup (please let me know if you
have a preference) and/or add a comment to clarify.

Best,

Mike
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