re: [PATCH] squashfs: enhance parallel I/O

[Phillip Lougher <phillip@lougher.demon.co.uk>]

Hi Minchan,

Apologies for the lateness of this review, I had forgotten I'd not
send it.

Minchan Kim <minchan@kernel.org> wrote:
>Now squashfs have used for only one stream buffer for decompression
>so it hurts concurrent read performance so this patch supprts
>multiple decompressor to enhance performance concurrent I/O.
>
>Four 1G file dd read on KVM machine which has 2 CPU and 4G memory.
>
>dd if=test/test1.dat of=/dev/null &
>dd if=test/test2.dat of=/dev/null &
>dd if=test/test3.dat of=/dev/null &
>dd if=test/test4.dat of=/dev/null &
>
>old : 1m39s -> new : 9s
>
>This patch is based on [1].
>
>[1] Squashfs: Refactor decompressor interface and code
>
>Cc: Phillip Lougher <phillip@squashfs.org.uk>
>Signed-off-by: Minchan Kim <minchan@kernel.org>
>
>---
>fs/squashfs/Kconfig              |   12 +++
> fs/squashfs/Makefile             |    9 +-
> fs/squashfs/decompressor_multi.c |  194 ++++++++++++++++++++++++++++++++++++++
> 3 files changed, 214 insertions(+), 1 deletion(-)
> create mode 100644 fs/squashfs/decompressor_multi.c
>
>diff --git a/fs/squashfs/Kconfig b/fs/squashfs/Kconfig
>index c70111e..7a580d0 100644
>--- a/fs/squashfs/Kconfig
>+++ b/fs/squashfs/Kconfig
>@@ -63,6 +63,18 @@ config SQUASHFS_LZO
>
> 	  If unsure, say N.
>
>+config SQUASHFS_MULTI_DECOMPRESSOR

Small alterations to the English, I don't like doing this because
English is probably a foreign language to you, and my Korean is non-existent!
but, this is user visible and you did say you wanted review !

>+	bool "Use multiple decompressor for handling concurrent I/O"

bool "Use multiple decompressors for handling parallel I/O"

Concurrent is subtly different to parallel, and you use parallel in
the following description.


>+	depends on SQUASHFS
>+	help
>+	  By default Squashfs uses a compressor for data but it gives
>+	  poor performance on parallel I/O workload of multiple CPU
>+	  mahchine due to waitting a compressor available.
>+

By default Squashfs uses a single decompressor but it gives
poor performance on parallel I/O workloads when using multiple CPU
machines due to waiting on decompressor availability.

>+	  If workload has parallel I/O and your system has enough memory,
>+	  this option may improve overall I/O performance.
>+	  If unsure, say N.
>+

If you have a parallel I/O workload and your system has enough memory,
using this option may improve overall I/O performance.

If unsure, say N.
>+
> config SQUASHFS_XZ
> 	bool "Include support for XZ compressed file systems"
> 	depends on SQUASHFS
>diff --git a/fs/squashfs/Makefile b/fs/squashfs/Makefile
>index c223c84..dfebc3b 100644
>--- a/fs/squashfs/Makefile
>+++ b/fs/squashfs/Makefile
>@@ -4,8 +4,15 @@
>
> obj-$(CONFIG_SQUASHFS) += squashfs.o
> squashfs-y += block.o cache.o dir.o export.o file.o fragment.o id.o inode.o
>-squashfs-y += namei.o super.o symlink.o decompressor.o decompressor_single.o
>+squashfs-y += namei.o super.o symlink.o decompressor.o
>+
> squashfs-$(CONFIG_SQUASHFS_XATTR) += xattr.o xattr_id.o
> squashfs-$(CONFIG_SQUASHFS_LZO) += lzo_wrapper.o
> squashfs-$(CONFIG_SQUASHFS_XZ) += xz_wrapper.o
> squashfs-$(CONFIG_SQUASHFS_ZLIB) += zlib_wrapper.o
>+
>+ifdef CONFIG_SQUASHFS_MULTI_DECOMPRESSOR
>+	squashfs-y		+= decompressor_multi.o
>+else
>+	squashfs-y		+= decompressor_single.o
>+endif
>diff --git a/fs/squashfs/decompressor_multi.c b/fs/squashfs/decompressor_multi.c
>new file mode 100644
>index 0000000..23f1e94
>--- /dev/null
>+++ b/fs/squashfs/decompressor_multi.c
>@@ -0,0 +1,194 @@
>+/*
>+ *  Copyright (c) 2013
>+ *  Minchan Kim <minchan@kernel.org>
>+ *
>+ *  This work is licensed under the terms of the GNU GPL, version 2. See
>+ *  the COPYING file in the top-level directory.
>+ */
>+#include <linux/types.h>
>+#include <linux/mutex.h>
>+#include <linux/slab.h>
>+#include <linux/buffer_head.h>
>+#include <linux/sched.h>
>+#include <linux/wait.h>
>+#include <linux/cpumask.h>
>+
>+#include "squashfs_fs.h"
>+#include "squashfs_fs_sb.h"
>+#include "decompressor.h"
>+#include "squashfs.h"
>+
>+/*
>+ * This file implements multi-threaded decompression in the
>+ * decompressor framework
>+ */
>+
>+
>+/*
>+ * The reason that multiply two is that a CPU can request new I/O
>+ * while it is waitting previous request.

s/waitting/waiting/

The English in the comments is understandable, and not user-visible, and
so I won't change it, except for spelling mistakes ...

>+ */
>+#define MAX_DECOMPRESSOR	(num_online_cpus() * 2)
>+
>+
>+int squashfs_max_decompressors(void)
>+{
>+	return MAX_DECOMPRESSOR;
>+}
>+
>+
>+struct squashfs_stream {
>+	void			*comp_opts;
>+	struct list_head	strm_list;
>+	struct mutex		mutex;
>+	int			avail_comp;
>+	wait_queue_head_t	wait;
>+};
>+
>+
>+struct comp_stream {
>+	void *stream;
>+	struct list_head list;
>+};
>+

One general small nitpick, you use "comp" to name things, comp_stream,
avail_comp etc.  But, as this is a decompressor, it should more correctly
be decomp...

(note comp_opts is not decomp_opts because it is the compression options
selected by the user at compression time) ...

>+
>+static void put_comp_stream(struct comp_stream *comp_strm,
>+				struct squashfs_stream *stream)
>+{
>+	mutex_lock(&stream->mutex);
>+	list_add(&comp_strm->list, &stream->strm_list);
>+	mutex_unlock(&stream->mutex);
>+	wake_up(&stream->wait);
>+}
>+
>+void *squashfs_decompressor_create(struct squashfs_sb_info *msblk,
>+				void *comp_opts)
>+{
>+	struct squashfs_stream *stream;
>+	struct comp_stream *comp_strm = NULL;
>+	int err = -ENOMEM;
>+
>+	stream = kzalloc(sizeof(*stream), GFP_KERNEL);
>+	if (!stream)
>+		goto out;
>+
>+	stream->comp_opts = comp_opts;
>+	mutex_init(&stream->mutex);
>+	INIT_LIST_HEAD(&stream->strm_list);
>+	init_waitqueue_head(&stream->wait);
>+
>+	comp_strm = kmalloc(sizeof(*comp_strm), GFP_KERNEL);
>+	if (!comp_strm)
>+		goto out;
>+
>+	comp_strm->stream = msblk->decompressor->init(msblk,
>+						stream->comp_opts);
>+	if (IS_ERR(comp_strm->stream)) {
>+		err = PTR_ERR(comp_strm->stream);
>+		goto out;
>+	}
>+
>+	list_add(&comp_strm->list, &stream->strm_list);
>+	stream->avail_comp = 1;

I assume you're always creating a decompressor here (rather than
setting it to 0, and allocating the first one in get_comp_stream) to
ensure there's always at least one decompressor available going into
get_comp_stream()?  So if decompressor creation fails in
get_comp_stream() we can always fall back to using the decompressors
already allocated, because we know there will always be at least one?

Maybe a comment to that effect? To show creating a decompressor here
and setting this to one is deliberate....  This will prevent others
from thinking they can "optimise" the code by having the first decompressor
created in get_comp_stream()!

/* ensure we have at least one decompressor in get_comp_stream() */ ?

>+	return stream;
>+
>+out:
>+	kfree(comp_strm);
>+	kfree(stream);
>+	return ERR_PTR(err);
>+}
>+
>+
>+void squashfs_decompressor_destroy(struct squashfs_sb_info *msblk)
>+{
>+	struct squashfs_stream *stream = msblk->stream;
>+	if (stream) {
>+		struct comp_stream *comp_strm;
>+
>+		while (!list_empty(&stream->strm_list)) {
>+			comp_strm = list_entry(stream->strm_list.prev,
>+						struct comp_stream, list);
>+			list_del(&comp_strm->list);
>+			msblk->decompressor->free(comp_strm->stream);
>+			kfree(comp_strm);
>+			stream->avail_comp--;
>+		}
>+	}
>+
>+	WARN_ON(stream->avail_comp);
>+	kfree(stream->comp_opts);
>+	kfree(stream);
>+}
>+
>+
>+static struct comp_stream *get_comp_stream(struct squashfs_sb_info *msblk,
>+					struct squashfs_stream *stream)
>+{
>+	struct comp_stream *comp_strm;
>+
>+	while (1) {
>+		mutex_lock(&stream->mutex);
>+
>+		/* There is available comp_stream */
>+		if (!list_empty(&stream->strm_list)) {
>+			comp_strm = list_entry(stream->strm_list.prev,
>+				struct comp_stream, list);
>+			list_del(&comp_strm->list);
>+			mutex_unlock(&stream->mutex);
>+			break;
>+		}
>+
>+		/*
>+		 * If there is no available comp and already full,
>+		 * let's wait for releasing comp from other users.
>+		 */
>+		if (stream->avail_comp >= MAX_DECOMPRESSOR)
>+			goto wait;
>+
>+		/* Let's allocate new comp */
>+		comp_strm = kmalloc(sizeof(*comp_strm), GFP_KERNEL);
>+		if (!comp_strm)
>+			goto wait;
>+
>+		comp_strm->stream = msblk->decompressor->init(msblk,
>+						stream->comp_opts);
>+		if (IS_ERR(comp_strm->stream)) {
>+			kfree(comp_strm);
>+			goto wait;
>+		}
>+
>+		stream->avail_comp++;
>+		WARN_ON(stream->avail_comp > MAX_DECOMPRESSOR);
>+
>+		mutex_unlock(&stream->mutex);
>+		break;
>+wait:
>+		/*
>+		 * If system memory is tough, let's for other's
>+		 * releasing instead of hurting VM because it could
>+		 * make page cache thrashing.
>+		 */


>+		mutex_unlock(&stream->mutex);
>+		wait_event(stream->wait,
>+			!list_empty(&stream->strm_list));
>+	}
>+
>+	return comp_strm;
>+}
>+
>+
>+int squashfs_decompress(struct squashfs_sb_info *msblk,
>+	void **buffer, struct buffer_head **bh, int b, int offset, int length,
>+	int srclength, int pages)
>+{

The interface here is changed by the introduction of the page
handler abstraction...

I can apply your patch after the refactoring patch and before the
"Squashfs: Directly decompress into the page cache for file" patch-set
and fix up the code here....  Or you can fix up the code?  I'm
happy to do either, whatever you prefer.

Thanks

Phillip


>+	int res;
>+	struct squashfs_stream *stream = msblk->stream;
>+	struct comp_stream *comp_stream = get_comp_stream(msblk, stream);
>+	res = msblk->decompressor->decompress(msblk, comp_stream->stream,
>+		buffer, bh, b, offset, length, srclength, pages);
>+	put_comp_stream(comp_stream, stream);
>+	if (res < 0)
>+		ERROR("%s decompression failed, data probably corrupt\n",
>+			msblk->decompressor->name);
>+	return res;
>+}