netdev - Re: [RFC/PATCH 2/3] UDP memory usage accounting: accounting unit and variable

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Message-ID: <p734pho9nb9.fsf@bingen.suse.de>
Date:	21 Sep 2007 15:10:18 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Satoshi OSHIMA <satoshi.oshima.fk@...achi.com>
Cc:	netdev@...r.kernel.org, haoki@...hat.com,
	吉藤英明 
	<yoshfuji@...ux-ipv6.org>
Subject: Re: [RFC/PATCH 2/3] UDP memory usage accounting: accounting unit and variable

Satoshi OSHIMA <satoshi.oshima.fk@...achi.com> writes:

> This patch introduces global variable for UDP memory accounting.
> The unit is page.

The global variable doesn't seem to be very MP scalable, especially
if you change it for each packet. This will be a very hot cache line,
in the worst case bouncing around a large machine.

Possible alternatives:
- Per CPU variables
- You only change the global on socket creation time (by pre allocating a large
amount) or when the system comes under memory pressure.
- Batching of the global updates for multiple packets [that's a variant
of the previous one, might be still too costly though]

Also for such variables it's usually good to cache line pad them on SMP
to avoid false sharing with something else.

-Andi
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