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Date: Wed, 19 May 2021 15:53:54 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>, Dave Taht
 <dave.taht@gmail.com>, Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
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 <xianting.tian@linux.alibaba.com>, Linux Kernel Network Developers
 <netdev@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
 virtualization <virtualization@lists.linux-foundation.org>, bloat
 <bloat@lists.bufferbloat.net>, "David S. Miller" <davem@davemloft.net>
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X-Mailman-Approved-At: Thu, 20 May 2021 11:31:23 -0400
Subject: Re: [Bloat] virtio_net: BQL?
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On Wed, 19 May 2021 16:31:10 +0200 Eric Dumazet wrote:
> On 5/18/21 1:00 AM, Stephen Hemminger wrote:
> > The Azure network driver (netvsc) also does not have BQL. Several years ago
> > I tried adding it but it benchmarked worse and there is the added complexity
> > of handling the accelerated networking VF path.
> 
> Note that NIC with many TX queues make BQL almost useless, only adding extra
> overhead.
> 
> We should probably make BQL something that can be manually turned on/off.

Ah, I've been pondering this. Are you thinking of a bit in
dev_queue->state? Not perfect, because with a careful driver design 
one can avoid most dev_queue accesses from the completion path.
It's still much better than recompiling the kernel to set BQL=n, tho.