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Date:	Fri, 24 Oct 2014 00:50:58 -0400 (EDT)
From:	David Miller <>
Subject: Re: [PATCHv2 net-next] xen-netfront: always keep the Rx ring full
 of requests

From: David Vrabel <>
Date: Wed, 22 Oct 2014 11:17:06 +0100

> A full Rx ring only requires 1 MiB of memory.  This is not enough
> memory that it is useful to dynamically scale the number of Rx
> requests in the ring based on traffic rates, because:
> a) Even the full 1 MiB is a tiny fraction of a typically modern Linux
>    VM (for example, the AWS micro instance still has 1 GiB of memory).
> b) Netfront would have used up to 1 MiB already even with moderate
>    data rates (there was no adjustment of target based on memory
>    pressure).
> c) Small VMs are going to typically have one VCPU and hence only one
>    queue.
> Keeping the ring full of Rx requests handles bursty traffic better
> than trying to converge on an optimal number of requests to keep
> filled.
> On a 4 core host, an iperf -P 64 -t 60 run from dom0 to a 4 VCPU guest
> improved from 5.1 Gbit/s to 5.6 Gbit/s.  Gains with more bursty
> traffic are expected to be higher.
> Signed-off-by: David Vrabel <>
> ---
> Changes in v2:
> - Keep rxbuf_* sysfs files.

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