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Subject[PATCH] O_STREAMING - flag for optimal streaming I/O
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Attached patch implements an O_STREAMING file I/O flag which enables
manual drop-behind of pages.

If the file has O_STREAMING set then the user has explicitly said "this
is streaming data, I know I will not revisit this, do not cache
anything". So we drop pages from the pagecache before our current
index. We have to fiddle a bit to get writes working since we do
write-behind but the logic is there and it works.

Some numbers. A simple streaming read to verify the pagecache effects:

Streaming 1GB Read (avg of many runs, mem=2GB):
O_STREAMING Wall time Change in Page Cache
Yes 25.58s 0
No 25.55s +835MB

Another read with much more VM pressure:

Streaming 1GB Read (avg of many runs, mem=8M)
O_STREAMING Wall time Change in Page Cache
Yes 25.76s 0
No 29.01s +1MB

And now the kicker:

Kernel compile (make -j2) and concurrent streaming I/O
(avg of two runs, mem=128M):
O_STREAMING Time to complete Kernel Compile
Yes 3m27.863s
No 4m15.818s

This is c/o Andrew Morton.

Patch is against 2.4.20-pre9. Why not 2.5? Because Andrew says we can
do better, perhaps with a real drop-behind heuristic. As 20 Oct looms
quite close, we shall see.

Robert Love

Implement O_STREAMING for streaming I/O for manual drop-behind of pages.

include/asm-arm/fcntl.h | 1
include/asm-i386/fcntl.h | 1
include/asm-mips/fcntl.h | 1
include/asm-ppc/fcntl.h | 1
include/asm-sh/fcntl.h | 1
mm/filemap.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++
6 files changed, 94 insertions(+)
diff -urN linux-2.4.20-pre9/include/asm-arm/fcntl.h linux/include/asm-arm/fcntl.h
--- linux-2.4.20-pre9/include/asm-arm/fcntl.h 2002-10-06 14:57:26.000000000 -0400
+++ linux/include/asm-arm/fcntl.h 2002-10-07 18:45:51.000000000 -0400
@@ -20,6 +20,7 @@
#define O_NOFOLLOW 0100000 /* don't follow links */
#define O_DIRECT 0200000 /* direct disk access hint - currently ignored */
#define O_LARGEFILE 0400000
+#define O_STREAMING 04000000 /* streaming access */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
diff -urN linux-2.4.20-pre9/include/asm-i386/fcntl.h linux/include/asm-i386/fcntl.h
--- linux-2.4.20-pre9/include/asm-i386/fcntl.h 2002-10-06 14:57:21.000000000 -0400
+++ linux/include/asm-i386/fcntl.h 2002-10-07 18:45:51.000000000 -0400
@@ -20,6 +20,7 @@
#define O_LARGEFILE 0100000
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
+#define O_STREAMING 04000000 /* streaming access */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
diff -urN linux-2.4.20-pre9/include/asm-mips/fcntl.h linux/include/asm-mips/fcntl.h
--- linux-2.4.20-pre9/include/asm-mips/fcntl.h 2002-10-06 14:57:21.000000000 -0400
+++ linux/include/asm-mips/fcntl.h 2002-10-07 18:45:51.000000000 -0400
@@ -26,6 +26,7 @@
#define O_DIRECT 0x8000 /* direct disk access hint */
#define O_DIRECTORY 0x10000 /* must be a directory */
#define O_NOFOLLOW 0x20000 /* don't follow links */
+#define O_STREAMING 0x400000 /* streaming access */

#define O_NDELAY O_NONBLOCK

diff -urN linux-2.4.20-pre9/include/asm-ppc/fcntl.h linux/include/asm-ppc/fcntl.h
--- linux-2.4.20-pre9/include/asm-ppc/fcntl.h 2002-10-06 14:57:22.000000000 -0400
+++ linux/include/asm-ppc/fcntl.h 2002-10-07 18:45:51.000000000 -0400
@@ -23,6 +23,7 @@
#define O_NOFOLLOW 0100000 /* don't follow links */
#define O_LARGEFILE 0200000
#define O_DIRECT 0400000 /* direct disk access hint */
+#define O_STREAMING 04000000 /* streaming access */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
diff -urN linux-2.4.20-pre9/include/asm-sh/fcntl.h linux/include/asm-sh/fcntl.h
--- linux-2.4.20-pre9/include/asm-sh/fcntl.h 2002-10-06 14:57:27.000000000 -0400
+++ linux/include/asm-sh/fcntl.h 2002-10-07 18:45:51.000000000 -0400
@@ -20,6 +20,7 @@
#define O_LARGEFILE 0100000
#define O_DIRECTORY 0200000 /* must be a directory */
#define O_NOFOLLOW 0400000 /* don't follow links */
+#define O_STREAMING 04000000 /* streaming access */

#define F_DUPFD 0 /* dup */
#define F_GETFD 1 /* get close_on_exec */
diff -urN linux-2.4.20-pre9/mm/filemap.c linux/mm/filemap.c
--- linux-2.4.20-pre9/mm/filemap.c 2002-10-06 14:57:20.000000000 -0400
+++ linux/mm/filemap.c 2002-10-07 18:45:51.000000000 -0400
@@ -1322,6 +1322,90 @@
SetPageReferenced(page);
}

+/**
+ * shrink_list - non-blockingly drop pages from the given cache list
+ * @mapping: the mapping from which we want to drop pages
+ * @list: which list (e.g. locked, dirty, clean)?
+ * @max_index: greatest index from which we will drop pages
+ */
+static unsigned long shrink_list(struct address_space *mapping,
+ struct list_head *list,
+ unsigned long max_index)
+{
+ struct list_head *curr = list->prev;
+ unsigned long nr_shrunk = 0;
+
+ spin_lock(&pagemap_lru_lock);
+ spin_lock(&pagecache_lock);
+
+ while ((curr != list)) {
+ struct page *page = list_entry(curr, struct page, list);
+
+ curr = curr->prev;
+
+ if (page->index > max_index)
+ continue;
+
+ if (PageDirty(page))
+ continue;
+
+ if (TryLockPage(page))
+ break;
+
+ if (page->buffers && !try_to_release_page(page, 0)) {
+ /* probably dirty buffers */
+ unlock_page(page);
+ break;
+ }
+
+ if (page_count(page) != 1) {
+ unlock_page(page);
+ continue;
+ }
+
+ __lru_cache_del(page);
+ __remove_inode_page(page);
+ unlock_page(page);
+ page_cache_release(page);
+ nr_shrunk++;
+ }
+
+ spin_unlock(&pagecache_lock);
+ spin_unlock(&pagemap_lru_lock);
+
+ return nr_shrunk;
+}
+
+/**
+ * shrink_pagecache - nonblockingly drop pages from the mapping.
+ * @file: the file we are doing I/O on
+ * @max_index: the maximum index from which we are willing to drop pages
+ *
+ * This is for O_STREAMING, which says "I am streaming data, I know I will not
+ * revisit this; do not cache anything".
+ *
+ * max_index allows us to only drop pages which are behind `index', to avoid
+ * trashing readahead.
+ */
+static unsigned long shrink_pagecache(struct file *file,
+ unsigned long max_index)
+{
+ struct address_space *mapping = file->f_dentry->d_inode->i_mapping;
+ unsigned long nr_locked, nr_clean, nr_dirty;
+
+ /*
+ * ensure we have a decent amount of work todo
+ */
+ if (mapping->nrpages < 256)
+ return 0;
+
+ nr_locked = shrink_list(mapping, &mapping->locked_pages, max_index);
+ nr_clean = shrink_list(mapping, &mapping->clean_pages, max_index);
+ nr_dirty = shrink_list(mapping, &mapping->dirty_pages, max_index);
+
+ return nr_locked + nr_clean + nr_dirty;
+}
+
/*
* This is a generic file read routine, and uses the
* inode->i_op->readpage() function for the actual low-level
@@ -1538,6 +1622,8 @@
filp->f_reada = 1;
if (cached_page)
page_cache_release(cached_page);
+ if (filp->f_flags & O_STREAMING)
+ shrink_pagecache(filp, index);
UPDATE_ATIME(inode);
}

@@ -3047,6 +3133,9 @@
if (file->f_flags & O_DIRECT)
goto o_direct;

+ if (file->f_flags & O_STREAMING)
+ shrink_pagecache(file, pos >> PAGE_CACHE_SHIFT);
+
do {
unsigned long index, offset;
long page_fault;
-
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