netdev - Re: [PATCH] vhost_net: use packet weight for rx handler, too

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Message-Id: <20180424.100250.2051235227878473641.davem@davemloft.net>
Date:   Tue, 24 Apr 2018 10:02:50 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     pabeni@...hat.com
Cc:     kvm@...r.kernel.org, haibinzhang@...cent.com, mst@...hat.com,
        jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH] vhost_net: use packet weight for rx handler, too

From: Paolo Abeni <pabeni@...hat.com>
Date: Tue, 24 Apr 2018 10:34:36 +0200

> Similar to commit a2ac99905f1e ("vhost-net: set packet weight of
> tx polling to 2 * vq size"), we need a packet-based limit for
> handler_rx, too - elsewhere, under rx flood with small packets,
> tx can be delayed for a very long time, even without busypolling.
> 
> The pkt limit applied to handle_rx must be the same applied by
> handle_tx, or we will get unfair scheduling between rx and tx.
> Tying such limit to the queue length makes it less effective for
> large queue length values and can introduce large process
> scheduler latencies, so a constant valued is used - likewise
> the existing bytes limit.
> 
> The selected limit has been validated with PVP[1] performance
> test with different queue sizes:
> 
> queue size		256	512	1024
> 
> baseline		366	354	362
> weight 128		715	723	670
> weight 256		740	745	733
> weight 512		600	460	583
> weight 1024		423	427	418
> 
> A packet weight of 256 gives peek performances in under all the
> tested scenarios.
> 
> No measurable regression in unidirectional performance tests has
> been detected.
> 
> [1] https://developers.redhat.com/blog/2017/06/05/measuring-and-comparing-open-vswitch-performance/
> 
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>

Applied to net-next, thanks.