bump maxthreads default.

bump the default MAXLWP to 4096 from 2048, and adjust the default
threads seen to be 2048 cur / 4096 max.  remove the linkage to
maxuprc entirely.

remove cpu_maxlwp() that isn't implemented anywhere.  instead,
grow the maxlwp for larger memory systems, picking 1 lwp per 1MiB
of ram, limited to 65535 like the system limit.

remove some magic numbers.


i've been having weird firefox issues for a few months now and
it turns out i was having pthread_create() failures and since
bumping the defaults i've had none of the recent issues.


Index: sys/kern/kern_lwp.c
===================================================================
RCS file: /cvsroot/src/sys/kern/kern_lwp.c,v
retrieving revision 1.248
diff -p -u -r1.248 kern_lwp.c
--- sys/kern/kern_lwp.c	9 Apr 2022 23:45:36 -0000	1.248
+++ sys/kern/kern_lwp.c	1 May 2022 20:40:22 -0000
@@ -294,6 +294,15 @@ struct lwp lwp0 __aligned(MIN_LWP_ALIGNM
 	.l_fd = &filedesc0,
 };
 
+static int
+lwp_maxlwp(void)
+{
+	/* Assume 1 LWP per 1MiB. */
+	uint64_t lwps_per = ctob(physmem) / (1024 * 1024);
+
+	return MAX(MIN(MAXMAXLWP, lwps_per), MAXLWP);
+}
+
 static int sysctl_kern_maxlwp(SYSCTLFN_PROTO);
 
 /*
@@ -313,9 +322,9 @@ sysctl_kern_maxlwp(SYSCTLFN_ARGS)
 	if (error || newp == NULL)
 		return error;
 
-	if (nmaxlwp < 0 || nmaxlwp >= 65536)
+	if (nmaxlwp < 0 || nmaxlwp >= MAXMAXLWP)
 		return EINVAL;
-	if (nmaxlwp > cpu_maxlwp())
+	if (nmaxlwp > lwp_maxlwp())
 		return EINVAL;
 	maxlwp = nmaxlwp;
 
@@ -350,7 +359,7 @@ lwpinit(void)
 	lwp_cache = pool_cache_init(sizeof(lwp_t), MIN_LWP_ALIGNMENT, 0,
 	    PR_PSERIALIZE, "lwppl", NULL, IPL_NONE, lwp_ctor, lwp_dtor, NULL);
 
-	maxlwp = cpu_maxlwp();
+	maxlwp = lwp_maxlwp();
 	sysctl_kern_lwp_setup();
 }
 
Index: sys/kern/kern_proc.c
===================================================================
RCS file: /cvsroot/src/sys/kern/kern_proc.c,v
retrieving revision 1.266
diff -p -u -r1.266 kern_proc.c
--- sys/kern/kern_proc.c	7 Apr 2022 19:33:38 -0000	1.266
+++ sys/kern/kern_proc.c	1 May 2022 20:40:22 -0000
@@ -539,7 +539,7 @@ proc0_init(void)
 	rlim[RLIMIT_MEMLOCK].rlim_cur = lim / 3;
 
 	rlim[RLIMIT_NTHR].rlim_max = maxlwp;
-	rlim[RLIMIT_NTHR].rlim_cur = maxlwp < maxuprc ? maxlwp : maxuprc;
+	rlim[RLIMIT_NTHR].rlim_cur = maxlwp / 2;
 
 	/* Note that default core name has zero length. */
 	limit0.pl_corename = defcorename;
Index: sys/sys/lwp.h
===================================================================
RCS file: /cvsroot/src/sys/sys/lwp.h,v
retrieving revision 1.215
diff -p -u -r1.215 lwp.h
--- sys/sys/lwp.h	9 Apr 2022 23:45:37 -0000	1.215
+++ sys/sys/lwp.h	1 May 2022 20:40:22 -0000
@@ -239,10 +239,10 @@ extern struct lwplist	alllwp;		/* List o
 extern lwp_t		lwp0;		/* LWP for proc0. */
 extern int		maxlwp __read_mostly;	/* max number of lwps */
 #ifndef MAXLWP
-#define	MAXLWP		2048
+#define	MAXLWP		4096		/* default max */
 #endif
-#ifndef	__HAVE_CPU_MAXLWP
-#define	cpu_maxlwp()	MAXLWP
+#ifndef MAXMAXLWP
+#define MAXMAXLWP	65535		/* absolute max */
 #endif
 #endif