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    Subject[PATCH] O_STREAMING - flag for optimal streaming I/O
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    Date
    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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