Date: Fri, 3 Nov 2006 02:28:17 +0100 (CET)
From: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
To: Eric Dumazet <dada1@cosmosbay.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: New filesystem for Linux
In-Reply-To: <454A76CC.6030003@cosmosbay.com>
Message-ID: <Pine.LNX.4.64.0611030223190.7781@artax.karlin.mff.cuni.cz>
References: <Pine.LNX.4.64.0611022221330.4104@artax.karlin.mff.cuni.cz>
 <454A76CC.6030003@cosmosbay.com>
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>> I have these questions:
>> 
>> * There is a rw semaphore that is locked for read for nearly all operations 
>> and locked for write only rarely. However locking for read causes cache 
>> line pingpong on SMP systems. Do you have an idea how to make it better?
>> 
>> It could be improved by making a semaphore for each CPU and locking for 
>> read only the CPU's semaphore and for write all semaphores. Or is there a 
>> better method?
>> 
>
> If you believe you need a semaphore for protecting a mostly read structure, 
> then RCU is certainly a good candidate. (ie no locked operation at all)

RCU is for non-blocking operation only.

Maybe it could be use for maintaining an unlocked variable that prevents 
readers from entering a critical section --- but waiting in a write for a 
quiscent state would hurt.

> The problem with a per_cpu biglock is that you may consume a lot of RAM for 
> big NR_CPUS. Count 32 KB per 'biglock' if NR_CPUS=1024
>
>> * This leads to another observation --- on i386 locking a semaphore is 2 
>> instructions, on x86_64 it is a call to two nested functions. Has it some 
>> reason or was it just implementator's laziness? Given the fact that locked 
>> instruction takes 16 ticks on Opteron (and can overlap about 2 ticks with 
>> other instructions), it would make sense to have optimized semaphores too.
>
> Hum, please dont use *lazy*, this could make Andi unhappy :)
>
> What are you calling semaphore exactly ?
> Did you read Documentation/mutex-design.txt ?

I see --- I could replace that semaphore with mutex. But rw-semaphores 
can't be replaces with them.

Mikulas
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