netdev - Re: [PATCH v5 2/2] virtio-net: use mtu size as buffer length for big packets

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Message-ID: <20220907152156-mutt-send-email-mst@kernel.org>
Date:   Wed, 7 Sep 2022 15:23:57 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Parav Pandit <parav@...dia.com>
Cc:     Gavin Li <gavinl@...dia.com>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "jesse.brandeburg@...el.com" <jesse.brandeburg@...el.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "sridhar.samudrala@...el.com" <sridhar.samudrala@...el.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "loseweigh@...il.com" <loseweigh@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        Gavi Teitz <gavi@...dia.com>,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
        Si-Wei Liu <si-wei.liu@...cle.com>
Subject: Re: [PATCH v5 2/2] virtio-net: use mtu size as buffer length for big
 packets

On Wed, Sep 07, 2022 at 07:18:06PM +0000, Parav Pandit wrote:
> 
> > From: Michael S. Tsirkin <mst@...hat.com>
> > Sent: Wednesday, September 7, 2022 3:12 PM
> 
> > > Because of shallow queue of 16 entries deep.
> > 
> > but why is the queue just 16 entries?
> I explained the calculation in [1] about 16 entries.
> 
> [1] https://lore.kernel.org/netdev/PH0PR12MB54812EC7F4711C1EA4CAA119DC419@PH0PR12MB5481.namprd12.prod.outlook.com/
> 
> > does the device not support indirect?
> >
> Yes, indirect feature bit is disabled on the device.

OK that explains it.

> > because with indirect you get 256 entries, with 16 s/g each.
> > 
> Sure. I explained below that indirect comes with 7x memory cost that is not desired.
> (Ignored the table memory allocation cost and extra latency).

Oh sure, it's a waste. I wonder what effect does the patch have
on bandwidth with indirect enabled though.


> Hence don't want to enable indirect in this scenario.
> This optimization also works with indirect with smaller indirect table.
> 
> > 
> > > With driver turn around time to repost buffers, device is idle without any
> > RQ buffers.
> > > With this improvement, device has 85 buffers instead of 16 to receive
> > packets.
> > >
> > > Enabling indirect in device can help at cost of 7x higher memory per VQ in
> > the guest VM.