Re: [PATCH] Mantaining turnstile aligned to 128 bytes in i386 CPUs

John Baldwin wrote:
> On Tuesday 16 January 2007 15:36, Attilio Rao wrote:
>> 2007/1/16, John Baldwin <jhb_at_freebsd.org>:
>>> On Tuesday 16 January 2007 11:51, Attilio Rao wrote:
>>>> 2006/7/28, Attilio Rao <attilio_at_freebsd.org>:
>>>>> After some thinking, I think it's better using init/fini methods
>>>>> (since they hide the sizeof(struct turnstile) with size parameter).
>>>>>
>>>>> Feedbacks and comments are welcome:
>>>>> http://users.gufi.org/~rookie/works/patches/uma_sync_init.diff
>>>> [CC'ed all the interested people]
>>>>
>>>> Even if a long time is passed I did some benchmarks based on ebizzy 
> tool.
>>>> This program claims to reproduce a real httpd server behaviour and is
>>>> used into the Linux world for benchmarks, AFAIK.
>>>> I think that results of the comparison on this patch is very
>>>> interesting, and I think it worths a commit :)
>>>> I think that results can be even better on a Xeon machine (I had no
>>>> chance to reproduce this on some of these).
>>>> (Results taken in consideration have been measured after some starts,
>>>> in order to minimize caching differences).
>>>>
>>>> The patch:
>>>> http://users.gufi.org/~rookie/works/patches/ts-sq/ts-sq.diff
>>> Looks good.  Some minor nits are that in subr_turnstile.c in the comment I
>>> would say "a turnstile is allocated" rather than "a turnstile is got from 
> a
>>> specific UMA zone" as it reads a little bit clearer.  Also, I would
>>> say "Allocate a" rather than "Get a" for the two _alloc() functions.  
> Also,
>>> why not just use UMA_ALIGN_CACHE and make UMA_ALIGN_CACHE (128 - 1) on 
> i386
>>> and amd64 rather than adding a new UMA_ALIGN_SYNC?
>> I was thinking that in this way anyone who wants to replace the
>> syncronizing primitive boundary to an appropriate value can do it.
>> I just used UMA_ALIGN_CACHE as default value beacause I don't know the
>> better boundary (for syncronizing primitives) for other arches.
> 
> Is there a good reason to not cache-align synch primitives?  That is, why 
> would an arch not use cache-align?  Also, is there a reason to not update 
> UMA_ALIGN_CACHE on x86?
> 

If you always cache-line-align them, that also addresses the Intel
recommendation to always keep them from sharing cache lines.

Scott