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Message-ID: <445159FA.7070206@omniti.com>
Date: Thu, 27 Apr 2006 19:55:38 -0400
From: Theo Schlossnagle <jesus@omniti.com>
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To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Bruce Momjian <pgman@candle.pha.pa.us>, josh.berkus@sun.com,
	robert.lor@sun.com, PostgreSQL-patches <pgsql-patches@postgresql.org>
Subject: Re: [BUGS] BUG #2401: spinlocks not available on amd64
References: <200604272231.k3RMVYM28488@candle.pha.pa.us>
	<16483.1146178926@sss.pgh.pa.us>
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Tom Lane wrote:

>Bruce Momjian <pgman@candle.pha.pa.us> writes:
>  
>
>>Great, changes attached and applied.  I removed the solaris_i386 and
>>solaris_x86_64.s files and made just one solaris_x86.s.  I updated the
>>build system to use the new file, updated the macros, and added some
>>documentation on the approach.  Thanks.
>>    
>>
>
>BTW, on the replacement of ldstub with cas: according to what I find in
>google, cas does not exist on pre-V9 SPARCs.  Are we sure no one uses
>sparc v8 chips anymore?  Is there any actual advantage to making that
>change?
>  
>
This is true, and can be addressed with #defines to pull in the old old 
code.  Just to note, you can't run Solaris 10 or any future version of 
Solaris on sparcv8 either.  Sparcv8plus is 3bit sparc code and it does 
support the cas operation.  The instruction cas that operates on a word 
is more much efficient than the code that it replaced.  I'm don't own or 
have access to any sparc machines old enough to have that issue (even on 
my guest accounts at my alma mater).  If this is a concern, it seems 
more than reasonable to #elif the case in for that architecture in the 
sparc assembly file -- being open source, you'll no doubt alienate some 
dude who thinks its cool to run Postgres on his Sparc ELC.

I'd remind everyone that the spinlock stuff is entirely optional at 
build time.  I think it be more elegant approach to discontinue spinlock 
support on sparcv8 and older sparc architectures and just force them to 
use heavier weight locking mechanisms.  I "accidentally" did this on my 
Sol10 amd64 box before I realized what the build system did.

I also think it immensely useful to replace all of the tas subsystem 
with cas so that one could reliabily lock these atomics with the process 
id of the locker.  This would allow extremely good diagnostics in the 
event that a backend process is abruptly teminated while holding a log 
on that shm segment.  If the process ID was in there, it would then be a 
contition that is both diagnosable and recoverable.

I could likely provide the cas bits and pieces to do this stuff on 
linux/freebsd/mac os x and windows on x86/64 sparcv8plus/sparcv9 and PPC 
if you think that would be useful.

Best regards,

Theo