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Message-ID: <46FD009B.1010904@hitachi.com>
Date: Fri, 28 Sep 2007 22:24:43 +0900
From: Satoshi OSHIMA <satoshi.oshima.fk@...achi.com>
To: Andi Kleen <andi@...stfloor.org>
Cc: netdev@...r.kernel.org, haoki@...hat.com,
????
<yoshfuji@...ux-ipv6.org>,
Yumiko SUGITA <yumiko.sugita.yf@...achi.com>
Subject: Re: [RFC/PATCH 2/3] UDP memory usage accounting: accounting unit
and variable
Hi,
Thank you for your comment.
Andi Kleen wrote:
> 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.
I understand what you pointed out. But I think the accounting
method I'm proposing is very similar to TCP accounting and per
socket accounting.
How do you think of it?
> Possible alternatives:
> - Per CPU variables
I'm afraid that sockets and socket buffers are handled on
various CPUs. I mean that socket creation might be done
on CPU-A but socket receiving might be done on CPU-B.
And per CPU variables must be counted up when socket
cap is checked. I'm afraid that per CPU vaiables are
also costly enough.
> - 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.
I believe that memory usage accounting should be done accurately.
Currently I couldn't see how can we know the accurate memory
accounting only when the system is under memory pressure.
But I revised the patch to avoid some atomic operations.
If I could find the good way to avoid atomic operation more,
I will add it.
Satoshi Oshima
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