netdev - Re: [PATCH net-next 3/3] net: auto-tune mergeable rx buffer size for improved performance

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Date:	Wed, 8 Jan 2014 19:37:53 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Michael Dalton <mwdalton@...gle.com>
Cc:	Jason Wang <jasowang@...hat.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
	Eric Dumazet <edumazet@...gle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	lf-virt <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH net-next 3/3] net: auto-tune mergeable rx buffer size for
 improved performance

On Fri, Dec 27, 2013 at 01:41:28PM -0800, Michael Dalton wrote:
> I'm working on a followup patchset to address current feedback. I think
> it will be cleaner to do a debugfs implementation for per-receive queue
> packet buffer size exporting, so I'm trying that out.
> 
> On Thu, Dec 26, 2013 at 7:04 PM, Jason Wang <jasowang@...hat.com> wrote:
> > We can make this more accurate by using extra data structure to track
> > the real buf size and using it as token.
> 
> I agree -- we can do precise buffer total len tracking. Something like
> struct mergeable_packet_buffer_ctx {
>    void *buf;
>    unsigned int total_len;

Maybe make total_len long so size is a power of 2.

> };

Hmm this doubles VQ cache footprint.
In the past when I tried increasong cache footprint
this hurt performance measureable. It's just a suggestion though,
YMMV, if numbers are good we don't need to argue about this.

> 
> Each receive queue could have a pointer to an array of N buffer contexts,
> where N is queue size (kzalloc'd in init_vqs or similar). That would
> allow us to allocate all of our buffer context data at startup.
> 
> Would this be preferred to the current approach or is there another
> approach you would prefer? All other things being equal, having precise
> length tracking is advantageous, so I'm inclined to try this out and
> see how it goes.
> 
> I think this is a big design point - for example, if we have an extra
> buffer context structure, then per-receive queue frag allocators are not
> required for auto-tuning and we can reduce the number of patches in
> this patchset.

I'd be careful with adding even more stuff in
mergeable_packet_buffer_ctx for above reason.

> I'm happy to implement either way.  Thanks!
> 
> Best,
> 
> Mike
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