From 78a779a3834cf39d7c0bcd93a15824b29df947a3 Mon Sep 17 00:00:00 2001
From: Russ Cox <rsc@swtch.com>
Date: Sun, 20 Jul 2008 03:23:19 -0400
Subject: 9660srv: import from Plan 9

---
 src/cmd/9660srv/iobuf.c | 177 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 177 insertions(+)
 create mode 100644 src/cmd/9660srv/iobuf.c

(limited to 'src/cmd/9660srv/iobuf.c')

diff --git a/src/cmd/9660srv/iobuf.c b/src/cmd/9660srv/iobuf.c
new file mode 100644
index 00000000..30eafb3a
--- /dev/null
+++ b/src/cmd/9660srv/iobuf.c
@@ -0,0 +1,177 @@
+#include <u.h>
+#include <libc.h>
+#include <auth.h>
+#include <fcall.h>
+#include "dat.h"
+#include "fns.h"
+
+/*
+ * We used to use 100 i/o buffers of size 2kb (Sectorsize).
+ * Unfortunately, reading 2kb at a time often hopping around
+ * the disk doesn't let us get near the disk bandwidth.
+ *
+ * Based on a trace of iobuf address accesses taken while
+ * tarring up a Plan 9 distribution CD, we now use 16 128kb
+ * buffers.  This works for ISO9660 because data is required
+ * to be laid out contiguously; effectively we're doing agressive
+ * readahead.  Because the buffers are so big and the typical 
+ * disk accesses so concentrated, it's okay that we have so few
+ * of them.
+ *
+ * If this is used to access multiple discs at once, it's not clear
+ * how gracefully the scheme degrades, but I'm not convinced
+ * it's worth worrying about.		-rsc
+ */
+
+/* trying a larger value to get greater throughput - geoff */
+#define	BUFPERCLUST	256 /* sectors/cluster; was 64, 64*Sectorsize = 128kb */
+#define	NCLUST		16
+
+int nclust = NCLUST;
+
+static Ioclust*	iohead;
+static Ioclust*	iotail;
+
+static Ioclust*	getclust(Xdata*, long);
+static void	putclust(Ioclust*);
+static void	xread(Ioclust*);
+
+void
+iobuf_init(void)
+{
+	int i, j, n;
+	Ioclust *c;
+	Iobuf *b;
+	uchar *mem;
+
+	n = nclust*sizeof(Ioclust) +
+		nclust*BUFPERCLUST*(sizeof(Iobuf)+Sectorsize);
+	mem = sbrk(n);
+	if(mem == (void*)-1)
+		panic(0, "iobuf_init");
+	memset(mem, 0, n);
+
+	for(i=0; i<nclust; i++){
+		c = (Ioclust*)mem;
+		mem += sizeof(Ioclust);
+		c->addr = -1;
+		c->prev = iotail;
+		if(iotail)
+			iotail->next = c;
+		iotail = c;
+		if(iohead == nil)
+			iohead = c;
+
+		c->buf = (Iobuf*)mem;
+		mem += BUFPERCLUST*sizeof(Iobuf);
+		c->iobuf = mem;
+		mem += BUFPERCLUST*Sectorsize;
+		for(j=0; j<BUFPERCLUST; j++){
+			b = &c->buf[j];
+			b->clust = c;
+			b->addr = -1;
+			b->iobuf = c->iobuf+j*Sectorsize;
+		}
+	}
+}
+
+void
+purgebuf(Xdata *dev)
+{
+	Ioclust *p;
+
+	for(p=iohead; p!=nil; p=p->next)
+		if(p->dev == dev){
+			p->addr = -1;
+			p->busy = 0;
+		}
+}
+
+static Ioclust*
+getclust(Xdata *dev, long addr)
+{
+	Ioclust *c, *f;
+
+	f = nil;
+	for(c=iohead; c; c=c->next){
+		if(!c->busy)
+			f = c;
+		if(c->addr == addr && c->dev == dev){
+			c->busy++;
+			return c;
+		}
+	}
+
+	if(f == nil)
+		panic(0, "out of buffers");
+
+	f->addr = addr;
+	f->dev = dev;
+	f->busy++;
+	if(waserror()){
+		f->addr = -1;	/* stop caching */
+		putclust(f);
+		nexterror();
+	}
+	xread(f);
+	poperror();
+	return f;
+}
+
+static void
+putclust(Ioclust *c)
+{
+	if(c->busy <= 0)
+		panic(0, "putbuf");
+	c->busy--;
+
+	/* Link onto head for LRU */
+	if(c == iohead)
+		return;
+	c->prev->next = c->next;
+
+	if(c->next)
+		c->next->prev = c->prev;
+	else
+		iotail = c->prev;
+
+	c->prev = nil;
+	c->next = iohead;
+	iohead->prev = c;
+	iohead = c;
+}
+
+Iobuf*
+getbuf(Xdata *dev, ulong addr)
+{
+	int off;
+	Ioclust *c;
+
+	off = addr%BUFPERCLUST;
+	c = getclust(dev, addr - off);
+	if(c->nbuf < off){
+		c->busy--;
+		error("I/O read error");
+	}
+	return &c->buf[off];
+}
+
+void
+putbuf(Iobuf *b)
+{
+	putclust(b->clust);
+}
+
+static void
+xread(Ioclust *c)
+{
+	int n;
+	Xdata *dev;
+
+	dev = c->dev;
+	seek(dev->dev, (vlong)c->addr * Sectorsize, 0);
+	n = readn(dev->dev, c->iobuf, BUFPERCLUST*Sectorsize);
+	if(n < Sectorsize)
+		error("I/O read error");
+	c->nbuf = n/Sectorsize;
+}
-- 
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