linux-kernel - Re: [RFC][PATCH 1/7] Resource counters

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Message-ID: <m1fy8bwqps.fsf@ebiederm.dsl.xmission.com>
Date:	Sun, 11 Mar 2007 13:00:15 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Pavel Emelianov <xemul@...ru>
Cc:	balbir@...ibm.com, containers@...ts.osdl.org,
	Paul Menage <menage@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/7] Resource counters

Herbert Poetzl <herbert@...hfloor.at> writes:

>
> Linux-VServer does the accounting with atomic counters,
> so that works quite fine, just do the checks at the
> beginning of whatever resource allocation and the
> accounting once the resource is acquired ...

Atomic operations versus locks is only a granularity thing.
You still need the cache line which is the cost on SMP.

Are you using atomic_add_return or atomic_add_unless or
are you performing you actions in two separate steps which
is racy?  What I have seen indicates you are using a racy two separate
operation form.

>> If we'll remove failcnt this would look like
>>    while (atomic_cmpxchg(...))
>> which is also not that good.
>> 
>> Moreover - in RSS accounting patches I perform page list
>> manipulations under this lock, so this also saves one atomic op.
>
> it still hasn't been shown that this kind of RSS limit
> doesn't add big time overhead to normal operations
> (inside and outside of such a resource container)
>
> note that the 'usual' memory accounting is much more
> lightweight and serves similar purposes ...

Perhaps....

Eric
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