netdev - Re: [PATCH V4 net-next 0/6] ethtool: add support to set/get tx copybreak buf size and rx buf len

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Message-ID: <20211014194146.168ca52b@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 14 Oct 2021 19:41:46 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     Guangbin Huang <huangguangbin2@...wei.com>, davem@...emloft.net,
        mkubecek@...e.cz, andrew@...n.ch, amitc@...lanox.com,
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Subject: Re: [PATCH V4 net-next 0/6] ethtool: add support to set/get tx
 copybreak buf size and rx buf len

On Thu, 14 Oct 2021 15:14:20 +0200 Alexander Lobakin wrote:
> > Rx buf len is buffer length of each rx BD. Use ethtool -g command to get
> > it, and ethtool -G command to set it, examples are as follow:
> > 
> > 1. set rx buf len to 4096
> > $ ethtool -G eth1 rx-buf-len 4096
> > 
> > 2. get rx buf len
> > $ ethtool -g eth1
> > ...
> > RX Buf Len:     4096  
> 
> Isn't that supposed to be changed on changing MTU?
> And what if I set Rx buf len value lower than MTU? I see no checks
> as well.

Presumably the NIC does scatter.

> That means, do we _really_ need two new tunables?

nit let's not say tunable, "tunable" has an API meaning this one
is part of the ring param command.

The question of "why do we need this" seems fair tho - Guangbin,
can you share examples of workloads which benefit from 2k vs 4k?