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Re: [HACKERS] Re: [PATCHES] patches for 6.2.1p6

From: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
To: scrappy(at)hub(dot)org (The Hermit Hacker)
Cc: dg(at)illustra(dot)com, dz(at)cs(dot)unitn(dot)it, hackers(at)postgreSQL(dot)org
Subject: Re: [HACKERS] Re: [PATCHES] patches for 6.2.1p6
Date: 1998-03-17 03:20:48
Message-ID: 199803170320.WAA01257@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
> 
> On Mon, 16 Mar 1998, David Gould wrote:
> 
> > The generic way to do this is
> > 
> >     select( NULL_FDSET, NULL_FDSET, NULL_FDSET, &delaytime, NULL);
> > 
> > Delay time may be 0, but a random value between 0 and say 30 msec seems
> > to be optimal. Hard busy wait spinlocks cause huge performance problems with
> > heavily loaded systems and lots of postgres backends. Basically one backend
> > ends up with the lock and gets scheduled out holding it, every else queues
> > up busywaiting behind this one. But the backend holding the lock cannot
> > release it until all the other backeds waiting on the lock exhaust a full
> > timeslice busywaiting. Get 20 of these guys going (like on a busy website) and
> > the system pretty much stops doing any work at all.
> > 
> > I say we should get this in as soon as we can.
> 
> Can you submit an appropriate patch that can be included in the mega-patch
> to be created on Sunday?

Just a warning that this is not going to be easy.  We have OS-specific
code for spinlocks in include/storage/s_lock.h and
backend/storage/buffer/s_lock.c.  So each S_LOCK macro call has to have
its test-and-set logic de-coupled with its while-lock-fail-try-again
logic.  Most of them are easy, but some like VAX:

#define S_LOCK(addr)        __asm__("1: bbssi $0,(%0),1b": :"r"(addr))

are hard to de-couple.  Now, I did not know we supported NetBSD on VAX. 
Does it work, anyone?  Can I remove it?

This is going to be pretty tough to test on every platform we support,
so if it is done now, it will have to be done carefully.

-- 
Bruce Momjian                          |  830 Blythe Avenue
maillist(at)candle(dot)pha(dot)pa(dot)us              |  Drexel Hill, Pennsylvania 19026
  +  If your life is a hard drive,     |  (610) 353-9879(w)
  +  Christ can be your backup.        |  (610) 853-3000(h)

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