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

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Date:   Sat, 3 Oct 2020 14:43:13 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Miller <davem@...emloft.net>
Cc:     anant.thazhemadam@...il.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

On Fri, Oct 02, 2020 at 07:06:38PM -0700, David Miller wrote:
> 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.

Agree wholeheartedly.

-- 
MST