netdev - Re: [PATCH net-next 2/3] virtio-net: use per-receive queue page frag alloc for mergeable bufs

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Message-ID: <1389204587.26646.111.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 08 Jan 2014 10:09:47 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Michael Dalton <mwdalton@...gle.com>,
	Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	lf-virt <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH net-next 2/3] virtio-net: use per-receive queue page
 frag alloc for mergeable bufs

On Wed, 2014-01-08 at 19:21 +0200, Michael S. Tsirkin wrote:

> Basically yes, we could start dropping packets immediately
> once GFP_ATOMIC allocations fail and repost the buffer to host,
> and hope memory is available by the time we get the next interrupt.

> But we wanted host to have visibility into the fact that
> we are out of memory and packets are dropped, so we did not want to
> repost.

bufferbloat alert :)

> If we don't repost how do we know memory is finally available?
> We went for a timer based workqueue thing.
> What do you suggest?


In normal networking land, when a host A sends frames to host B,
nothing prevents A to pause the traffic to B if B is dropping packets
under stress.

A physical NIC do not use a workqueue to refill its RX queue but uses
the following strategy :

0) Pre filling of RX ring buffer with N frames. This can use GFP_KERNEL
   allocations with all needed (sleep/retry/shout) logic...
1) IRQ is handled.
2) Can we allocate a new buffer (GFP_ATOMIC) ?
   If yes, we accept the frame,
      and post the new buffer for the 'next frame'
   If no, we drop the frame and recycle the memory for next round.



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