Date: Fri, 7 May 1999 21:58:32 +0100 (BST) From: Doug Rabson <dfr@nlsystems.com> To: Matthew Dillon <dillon@apollo.backplane.com> Cc: current@FreeBSD.ORG Subject: Re: Debugging FreeBSD user threads with gdb Message-ID: <Pine.BSF.4.05.9905072154350.10400-100000@herring.nlsystems.com> In-Reply-To: <199905072036.NAA47016@apollo.backplane.com>
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On Fri, 7 May 1999, Matthew Dillon wrote:
> :> :--- uthread_create.c 1999/03/23 05:07:55 1.12
> :> :+++ uthread_create.c 1999/05/06 15:27:33
> :> :@@ -42,6 +42,8 @@
> :> : #include "pthread_private.h"
> :> : #include "libc_private.h"
> :> :
> :> :+static int next_tid = 1;
> :> :+
> :> : int
> :> : pthread_create(pthread_t * thread, const pthread_attr_t * attr,
> :> : void *(*start_routine) (void *), void *arg)
> :> :@@ -87,6 +89,7 @@
> :> : } else {
> :> : /* Initialise the thread structure: */
> :> : memset(new_thread, 0, sizeof(struct pthread));
> :> :+ new_thread->tid = next_tid++;
> :> :
> :> :Doug Rabson Mail: dfr@nlsystems.com
> :> :Nonlinear Systems Ltd. Phone: +44 181 442 9037
> :>
> :> Hmmm. tid is only an int and some programs which create and destroy
> :> threads a lot are almost certainly going to overflow it. 4 billion
> :> is not hard to reach. This can result in duplicate tid's.
> :
> :Didn't think of that. It gets a bit ugly inside gdb since gdb needs a
> :single int to encode the pid and thread id. I'm currently allowing 20
> :bits for the pid and 11 for the tid. Any ideas?
> :
> :--
> :Doug Rabson Mail: dfr@nlsystems.com
> :Nonlinear Systems Ltd. Phone: +44 181 442 9037
>
> Hmmm. 11 bits is only 2048 threads. You are going to have problems
> no matter what.
>
> You could have a pool of tid structures. A simple freelist. Pseudo
> code below. This would fix the unique-tid problem but it would not fix
> the 2048 thread limit with gdb.
I can probably increase the field size of the tid up to about 14 bits (we
only have pids going up to 100000 I think). I would rather not do this
amount of work inside uthread; perhaps it would be better to make the
uthread store a 64bit unique number and then compress it down to a 14 bit
tid inside gdb by keeping a hash table or something similar.
Its going to be pretty hard to debug a program with 16k active threads so
I think this limit should be ok.
--
Doug Rabson Mail: dfr@nlsystems.com
Nonlinear Systems Ltd. Phone: +44 181 442 9037
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