netdev - Re: [Linux-kernel-mentees][PATCH 0/2] reorder members of structures in virtio

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Date:   Fri, 02 Oct 2020 19:06:38 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     anant.thazhemadam@...il.com
Cc:     mst@...hat.com, jasowang@...hat.com, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, hawk@...nel.org,
        john.fastabend@...il.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [Linux-kernel-mentees][PATCH 0/2] reorder members of
 structures in virtio_net for optimization

From: Anant Thazhemadam <anant.thazhemadam@...il.com>
Date: Wed, 30 Sep 2020 10:47:20 +0530

> The structures virtnet_info and receive_queue have byte holes in 
> middle, and their members could do with some rearranging 
> (order-of-declaration wise) in order to overcome this.
> 
> Rearranging the members helps in:
>   * elimination the byte holes in the middle of the structures
>   * reduce the size of the structure (virtnet_info)
>   * have more members stored in one cache line (as opposed to 
>     unnecessarily crossing the cacheline boundary and spanning
>     different cachelines)
> 
> The analysis was performed using pahole.
> 
> These patches may be applied in any order.

What effects do these changes have on performance?

The cache locality for various TX and RX paths could be effected.

I'm not applying these patches without some data on the performance
impact.

Thank you.