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Date: Tue, 16 Nov 2004 07:41:31 +0800
From: David Xu <davidxu@freebsd.org>
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To: John Baldwin <jhb@freebsd.org>
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cc: Perforce Change Reviews <perforce@freebsd.org>
Subject: Re: PERFORCE change 65074 for review
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John Baldwin wrote:

>On Sunday 14 November 2004 12:13 am, David Xu wrote:
>  
>
>>http://perforce.freebsd.org/chv.cgi?CH=65074
>>
>>Change 65074 by davidxu@davidxu_alona on 2004/11/14 05:12:40
>>
>>	1. Fix a race between signal and umtx_unlock. a waiter
>>	   may be resumed by signal and left or exited, heavily
>>	   loaded test causes kernel to crash.
>>	2. Use distributed queue locks instead of single giant
>>	   lock.
>>
>>Affected files ...
>>
>>.. //depot/projects/davidxu_ksedbg/src/sys/kern/kern_umtx.c#4 edit
>>
>>Differences ...
>>
>>==== //depot/projects/davidxu_ksedbg/src/sys/kern/kern_umtx.c#4 (text+ko)
>>====
>>
>>@@ -49,25 +49,48 @@
>> 	pid_t		uq_pid;		/* Pid key component. */
>> };
>>
>> #define	UMTX_QUEUES	128
>> #define	UMTX_HASH(pid, umtx)						\
>>-    (((uintptr_t)pid + ((uintptr_t)umtx & ~65535)) % UMTX_QUEUES)
>>+    ((((uintptr_t)pid << 16) + ((uintptr_t)umtx & 65535)) % UMTX_QUEUES)
>>    
>>
>
>I'm curious why you changed the hash macro here?  Low order bits of pointers 
>tend to be zero due to alignment, so I think this will result in fewer 
>"useful" bits and more collisions and longer chains.
>
>  
>
FYI, I changed back to original hash code:
I simulate it under userland:

printf("thread lock: %p hash=%d\n", &thread->lock,
           ((getpid() + ((unsigned)(&thread->lock) & ~0xFFFF))) % 128);

get result:

thread lock: 0x804e014 hash=39
thread lock: 0x8053014 hash=39
thread lock: 0x8056014 hash=39
thread lock: 0x805a014 hash=39
thread lock: 0x805d014 hash=39
thread lock: 0x8060014 hash=39
thread lock: 0x8063014 hash=39
thread lock: 0x8067014 hash=39
thread lock: 0x806a014 hash=39
thread lock: 0x806d014 hash=39
thread lock: 0x8070014 hash=39

So my version put all locks on chain 0, the orignal version put all
locks on chain 39. :( Both seem bad algorithm.
Note it is my private version of libpthread using thr and umtx,
1:1 only.

David