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page: performance fixes

- fix showpage1 only decrementing proc counter once limit is reached;
this blocked having more than one loadpages process after NPROC calls,
since the next one has to wait until the last has exited
- allow procs to skip pages currently being loaded by others; this
forced processes to wait for each other at the same page
- bump NPROC from 4 to 8
- (hack) immediately fork a few times after adding all pages at
startup to force loading a batch of pages in parallel

--- a/sys/src/cmd/page.c

+++ b/sys/src/cmd/page.c

@@ -54,7 +54,7 @@

 char pagespool[] = "/tmp/pagespool.";

 enum {

-	NPROC = 4,

+	NPROC = 8,

 	NBUF = 8*1024,

 	NPATH = 1024,

 };

@@ -898,10 +898,6 @@

 {

 	int fd;

-	qlock(&lru);

-	llinkhead(p);

-	qunlock(&lru);

-

 	if(p->open != nil && p->image == nil){

 		fd = openpage(p);

 		if(fd >= 0){

@@ -951,7 +947,11 @@

 loadpages(Page *p, int oviewgen)

 {

 	while(p != nil && viewgen == oviewgen){

-		qlock(p);

+		qlock(&lru);

+		llinkhead(p);

+		qunlock(&lru);

+		if(!canqlock(p))

+			goto next;

 		loadpage(p);

 		if(viewgen != oviewgen){

 			unloadpage(p);

@@ -972,6 +972,7 @@

 			unlockdisplay(display);

 		}

 		qunlock(p);

+	next:

 		if(p != current && imemsize >= imemlimit)

 			break;		/* only one page ahead once we reach the limit */

 		if(forward < 0){

@@ -1309,16 +1310,17 @@

 	writeaddr(p, "/dev/label");

 	current = p;

 	oviewgen = viewgen;

+	if(nproc >= NPROC)

+		waitpid();

 	switch(rfork(RFPROC|RFMEM)){

 	case -1:

 		sysfatal("rfork: %r");

 	case 0:

 		loadpages(p, oviewgen);

+		nproc--;

 		exits(nil);

 	}

-	if(++nproc >= NPROC)

-		if(waitpid() > 0)

-			nproc--;

+	nproc++;

 }

 /* recursive display lock, called from main proc only */

@@ -1691,6 +1693,8 @@

 		addpage(root, "stdin", popenfile, strdup("/fd/0"), -1);

 	for(; *argv; argv++)

 		addpage(root, *argv, popenfile, strdup(*argv), -1);

+	for(i=0; i<NPROC/4; i++)	/* rice */

+		showpage1(current);

 	drawlock(1);

 	for(;;){