TTY: tty flip buffer optimisation.

Currently, free flip buffer (tty->buf.free) reserve memory for further used, only if driver send to ldisc less 257 bytes in one time.
If driver send more, flip buffer reserve(kmalloc()) and then free(kfree()) every chunk more 256 bytes every time.

This patch allow reserve more than 256 bytes chunks in free buffer. And have self-regulation chunk size ability(very useful for pty). Also we can control memory used(at the most TTY_BUFFER_MAX). And avoiding useless looking up buffer more then 256 byte size.

With this patch I get follow results:

ilya@serh:~/src/pty$ time ./bench_pty_buf 253
chunk = 253. loop = 3952569.
real	0m21.017s
user	0m0.436s
sys	0m17.749s

ilya@serh:~/src/pty$ time ./bench_pty_buf 255
chunk = 255. loop = 3921568.
real	0m21.276s
user	0m0.544s
sys	0m17.329s

ilya@serh:~/src/pty$ time ./bench_pty_buf 257
chunk = 257. loop = 3891050.
real	0m21.652s
user	0m0.512s
sys	0m18.593s

Signed-off-by Ilya Zykov <[email protected]>

---
diff -uprN -X ../../dontdiff a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c

drivers/tty/tty_buffer.c +16 -1

@@ -58,8 +58,21 @@ static struct tty_buffer *tty_buffer_all
 {
 	struct tty_buffer *p;
 -	if (tty->buf.memory_used + size > 65536)
-		return NULL;
+	if (tty->buf.memory_reserve + size > TTY_BUFFER_MAX) {
+		/* If possible, free small chunks
+				have been had total size = "size". */
+		if (tty->buf.memory_reserve - tty->buf.memory_used >= size)
+			while (tty->buf.memory_reserve + size >
+							TTY_BUFFER_MAX) {
+				p = tty->buf.free;
+				tty->buf.free = p->next;
+				tty->buf.memory_reserve -= p->size;
+				WARN_ON(tty->buf.memory_reserve < 0);
+				kfree(p);
+			}
+		else
+			return NULL;
+	}
 	p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
 	if (p == NULL)
 		return NULL;

@@ -71,6 +84,7 @@ static struct tty_buffer *tty_buffer_all
 	p->char_buf_ptr = (char *)(p->data);
 	p->flag_buf_ptr = (unsigned char *)p->char_buf_ptr + size;
 	tty->buf.memory_used += size;
+	tty->buf.memory_reserve += size;
 	return p;
 }
 @@ -91,11 +105,12 @@ static void tty_buffer_free(struct tty_s

drivers/tty/tty_buffer.c +5 -2

 	tty->buf.memory_used -= b->size;
 	WARN_ON(tty->buf.memory_used < 0);
 -	if (b->size >= 512)
-		kfree(b);
-	else {
+	if (tty->buf.free != NULL) {
 		b->next = tty->buf.free;
 		tty->buf.free = b;
+	} else {
+		tty->buf.free = b;
+		b->next = NULL;
 	}
 }
 @@ -517,6 +532,7 @@ void tty_buffer_init(struct tty_struct *
 	tty->buf.tail = NULL;
 	tty->buf.free = NULL;
 	tty->buf.memory_used = 0;
+	tty->buf.memory_reserve = 0;
 	INIT_WORK(&tty->buf.work, flush_to_ldisc);
 }
 diff -uprN -X ../../dontdiff a/include/linux/tty.h b/include/linux/tty.h

include/linux/tty.h +1 -1

@@ -82,7 +82,7 @@ struct tty_buffer {
  */
  #define TTY_BUFFER_PAGE	(((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)
-
+#define TTY_BUFFER_MAX		65536
  struct tty_bufhead {
 	struct work_struct work;

Hi, the results are indeed nice. However is there any *real* load other than this tailor-made microbenchmark where the added code complexity is worth it?

Quoted 3 lines

Hi, the results are indeed nice. However is there any *real* load other than this tailor-made microbenchmark where the added code complexity is worth it?

I'm wondering if we need the complexity in the first place. Certainly 256 does seem a bit small for pty/tty traffic. A 'real world' benchmark would be an ls -lR / on a machine with a fast graphics card or in console mode

ie

ls -lR / # prime cache
time ls -lR /

and there are cases where people do a lot of traffic over a pty like this so I don't think it's entirely fake.

I don't like the complexity but we could certainly go from using 256 byte buffers to "tty->buf.bufsize" and make it configurable without that complexity.

Alan

For avoid complexity we need remove free buffer at all.
And use kmalloc()-kfree() for every chunk. Don't need tty_buffer_find(). It will be fast and easy.