linux-kernel - Re: [Bloat] virtio

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Message-ID: <20210519155354.4438565e@kicinski-fedora-PC1C0HJN>
Date:   Wed, 19 May 2021 15:53:54 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Dave Taht <dave.taht@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Xianting Tian <xianting.tian@...ux.alibaba.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        bloat <bloat@...ts.bufferbloat.net>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [Bloat] virtio_net: BQL?

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.