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Re: [Qemu-devel] [ RFC Patch v4 2/3] virtio-net rsc: support coalescing

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [ RFC Patch v4 2/3] virtio-net rsc: support coalescing ipv4 tcp traffic
Date: Tue, 12 Apr 2016 11:23:28 +0300

On Mon, Apr 04, 2016 at 03:25:55AM +0800, address@hidden wrote:
> From: Wei Xu <address@hidden>
> All the data packets in a tcp connection will be cached to a big buffer
> in every receive interval, and will be sent out via a timer, the
> 'virtio_net_rsc_timeout' controls the interval, the value will influent the
> performance and response of tcp connection extremely, 50000(50us) is a
> experience value to gain a performance improvement, since the whql test
> sends packets every 100us, so '300000(300us)' can pass the test case,
> this is also the default value, it's gonna to be tunable.
> The timer will only be triggered if the packets pool is not empty,
> and it'll drain off all the cached packets
> 'NetRscChain' is used to save the segments of different protocols in a
> VirtIONet device.
> The main handler of TCP includes TCP window update, duplicated ACK check
> and the real data coalescing if the new segment passed sanity check
> and is identified as an 'wanted' one.
> An 'wanted' segment means:
> 1. Segment is within current window and the sequence is the expected one.
> 2. ACK of the segment is in the valid window.
> 3. If the ACK in the segment is a duplicated one, then it must less than 2,
>    this is to notify upper layer TCP starting retransmission due to the spec.
> Sanity check includes:
> 1. Incorrect version in IP header
> 2. IP options & IP fragment
> 3. Not a TCP packets
> 4. Sanity size check to prevent buffer overflow attack.
> There maybe more cases should be considered such as ip identification other
> flags, while it broke the test because windows set it to the same even it's
> not a fragment.
> Normally it includes 2 typical ways to handle a TCP control flag, 'bypass'
> and 'finalize', 'bypass' means should be sent out directly, and 'finalize'
> means the packets should also be bypassed, and this should be done
> after searching for the same connection packets in the pool and sending
> all of them out, this is to avoid out of data.
> All the 'SYN' packets will be bypassed since this always begin a new'
> connection, other flags such 'FIN/RST' will trigger a finalization, because
> this normally happens upon a connection is going to be closed, an 'URG' packet
> also finalize current coalescing unit.
> Statistics can be used to monitor the basic coalescing status, the 'out of 
> order'
> and 'out of window' means how many retransmitting packets, thus describe the
> performance intuitively.
> Signed-off-by: Wei Xu <address@hidden>

Yan pointed out that VIRTIO_NET_HDR_GSO_* is never used in this patch,
nor is gso_type ever set in virtio net header.

This seems strange.


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