netdev - Re: [PATCH net-next 1/3] net: allow > 0 order atomic page alloc in skb_page_frag

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Date:	Fri, 3 Jan 2014 18:27:11 -0500
From:	Debabrata Banerjee <dbavatar@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Michael Dalton <mwdalton@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>, mst@...hat.com,
	jasowang@...hat.com, virtualization@...ts.linux-foundation.org,
	"Banerjee, Debabrata" <dbanerje@...mai.com>, jbaron@...mai.com,
	Joshua Hunt <johunt@...mai.com>
Subject: Re: [PATCH net-next 1/3] net: allow > 0 order atomic page alloc in skb_page_frag_refill

On Fri, Jan 3, 2014 at 5:54 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> This is in GFP_ATOMIC cases, I dont think it can ever start compaction.

I think that's right I probably finally got it back to normal behavior
with order-0 allocations.

>
> It seems that you shoot the messenger : If memory is fragmented, then
> one order-1 allocation is going to start compaction.
>
> It can be a simple fork().
>
> If your workload never fork(), then yes, you never needed compaction.
>

Sure but the rate of network packets in and out and subsequent
allocations would be more equivalent to a fork bomb than normal
forking. I understand mm should work more sanely in this scenario but
at the same time we see a bad regression with this code, I see we're
not alone.

>
> We are not trying to optimize the kernel behavior for hosts in deep
> memory pressure.

We're leaving about half for the kernel so I wouldn't call it "deep".
Any server application that is using page cache and mlocked memory
will run into similar issues.

> It doesn't really matter to say that which memory allocation triggered
> compaction, which is a normal step in mm layer.
>
> If you believe its badly done, you should ask to mm guys to fix/improve
> it, not netdev...
>
> Using order-3 pages in TCP stack improves performance for 99% of the
> hosts, there might be something wrong on your side ?
>

Having lots of memory mlocked is bad right now yes, but not
necessarily an uncommon scenario. We're handing mm an almost
intractable problem. I see compaction of mlocked pages has been
discussed a few times over there, but no patch has actually made it
in.

-Debabrata
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