netdev - Re: [PATCH v2 net] ibmveth: Reduce maximum tx queues to 8

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Date:   Fri, 4 Nov 2022 10:59:55 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Nick Child <nnac123@...ux.ibm.com>
Cc:     netdev@...r.kernel.org, nick.child@....com, bjking1@...ux.ibm.com,
        ricklind@...ibm.com, dave.taht@...il.com
Subject: Re: [PATCH v2 net] ibmveth: Reduce maximum tx queues to 8

On Fri, 4 Nov 2022 09:06:02 -0500 Nick Child wrote:
> On 11/3/22 22:59, Jakub Kicinski wrote:
> > On Wed,  2 Nov 2022 13:38:37 -0500 Nick Child wrote:  
> >> Previously, the maximum number of transmit queues allowed was 16. Due to
> >> resource concerns, limit to 8 queues instead.
> >>
> >> Since the driver is virtualized away from the physical NIC, the purpose
> >> of multiple queues is purely to allow for parallel calls to the
> >> hypervisor. Therefore, there is no noticeable effect on performance by
> >> reducing queue count to 8.  
> > 
> > I'm not sure if that's the point Dave was making but we should be
> > influencing the default, not the MAX. Why limit the MAX?  
> 
> The MAX is always allocated in the drivers probe function. In the 
> drivers open and ethtool-set-channels functions we set 
> real_num_tx_queues. So the number of allocated queues is always MAX
> but the number of queues actually in use may differ and can be set by 
> the user.
> I hope this explains. Otherwise, please let me know.

Perhaps I don't understand the worry. Is allowing 16 queues a problem
because it limits how many instances the hypervisor can support?
Or is the concern coming from your recent work on BQL and having many
queues exacerbating buffer bloat?