Re: [patch] SMP alternatives

[Pádraig Brady <P@draigBrady.com>]

Linus Torvalds wrote:

>On Wed, 23 Nov 2005, Daniel Jacobowitz wrote:
>  
>
>>Why should we use a silicon based solution for this, when I posit that
>>there are simpler and equally effective userspace solutions?
>>    
>>
>
>Name them.
>
>In user space, doing things like clever run-time linking things is 
>actually horribly bad. It causes COW faults at startup, and/or makes the 
>compiler have to do indirections unnecessarily.  Both of which actually 
>make caches less effective, because now processes that really effectively 
>do have exactly the same contents have them in different pages.
>
>The other alternative (which apparently glibc actually does use) is to 
>dynamically branch over the lock prefixes, which actually works better: 
>it's more work dynamically, but it's much cheaper from a startup 
>standpoint and there's no memory duplication, so while it is the "stupid" 
>approach, it's actually better than the clever one.
>  
>
Just a note to say glibc is getting better wrt to locking.
Compare the results and trival test program here:
http://lkml.org/lkml/2001/12/7/75
That showed that for glibc 2.2.4, getc & putc
were 669% slower than the unlocked versions.

4 years later and with 2.3.5-1ubuntu1, getc & putc
are only 230% slower than the unlocked versions:

$ dd bs=1MB count=100 if=/dev/zero | ./locked >/dev/null
100000000 bytes transferred in 3.709362 seconds (26958813 bytes/sec)
$ dd bs=1MB count=100 if=/dev/zero | ./unlocked >/dev/null
100000000 bytes transferred in 1.602427 seconds (62405339 bytes/sec)

Pádraig.
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