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Date: Thu, 14 Aug 2008 15:16:05 -0700
From: Mingming Cao <cmm@...ibm.com>
To: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc: tytso@....edu, sandeen@...hat.com, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Fix small file fragmentation
在 2008-08-14四的 23:14 +0530，Aneesh Kumar K.V写道：
> mballoc small file block allocation use per cpu prealloc
> space. Use goal block when searching for the right prealloc
> space. Also make sure ext4_da_writepages tries to write
> all the pages for small files in single attempt
>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
> ---
> fs/ext4/inode.c | 21 +++++++++++++++------
> fs/ext4/mballoc.c | 44 +++++++++++++++++++++++++++++++++++++-------
> fs/inode.c | 1 +
> 3 files changed, 53 insertions(+), 13 deletions(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index e144896..0b34998 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2318,13 +2318,12 @@ static int ext4_writepages_trans_blocks(struct inode *inode)
> static int ext4_da_writepages(struct address_space *mapping,
> struct writeback_control *wbc)
> {
> - struct inode *inode = mapping->host;
> handle_t *handle = NULL;
> - int needed_blocks;
> - int ret = 0;
> - long to_write;
> loff_t range_start = 0;
> - long pages_skipped = 0;
> + struct inode *inode = mapping->host;
> + int needed_blocks, ret = 0, nr_to_writebump = 0;
> + long to_write, pages_skipped = 0;
> + struct ext4_sb_info *sbi = EXT4_SB(mapping->host->i_sb);
>
> /*
> * No pages to write? This is mainly a kludge to avoid starting
> @@ -2333,6 +2332,16 @@ static int ext4_da_writepages(struct address_space *mapping,
> */
> if (!mapping->nrpages || !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
> return 0;
> + /*
> + * Make sure nr_to_write is >= sbi->s_mb_stream_request
> + * This make sure small files blocks are allocated in
> + * single attempt. This ensure that small files
> + * get less fragmented.
> + */
> + if (wbc->nr_to_write < sbi->s_mb_stream_request) {
> + nr_to_writebump = sbi->s_mb_stream_request - wbc->nr_to_write;
> + wbc->nr_to_write = sbi->s_mb_stream_request;
> + }
>
do_writepages() could be called with wbc with a specified range, is it
okay forces da writepages to flush at last 16
pages(sbi->s_mb_stream_request) all the time, which is more than what
the caller asked for?
I assume you trying to address the fragmentation issue with small
request for da_writepages() discussed in previous email? (A little more
description will be helpful here:))
> if (!wbc->range_cyclic)
> /*
> @@ -2413,7 +2422,7 @@ static int ext4_da_writepages(struct address_space *mapping,
> }
>
> out_writepages:
> - wbc->nr_to_write = to_write;
> + wbc->nr_to_write = to_write - nr_to_writebump;
> wbc->range_start = range_start;
> return ret;
> }
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index b14a7c7..1afcb11 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -3286,6 +3286,29 @@ static void ext4_mb_use_group_pa(struct ext4_allocation_context *ac,
> mb_debug("use %u/%u from group pa %p\n", pa->pa_lstart-len, len, pa);
> }
>
> +static struct ext4_prealloc_space *
> +ext4_mb_check_group_pa(ext4_fsblk_t goal_block,
> + struct ext4_prealloc_space *pa,
> + struct ext4_prealloc_space *cpa)
> +{
> + ext4_fsblk_t cur_distance, new_distance;
> +
> + if (cpa == NULL) {
> + atomic_inc(&pa->pa_count);
> + return pa;
> + }
> + cur_distance = abs(goal_block - cpa->pa_pstart);
> + new_distance = abs(goal_block - pa->pa_pstart);
> +
> + if (cur_distance < new_distance)
> + return cpa;
> +
> + /* drop the previous reference */
> + atomic_dec(&cpa->pa_count);
> + atomic_inc(&pa->pa_count);
> + return pa;
> +}
> +
> /*
> * search goal blocks in preallocated space
> */
> @@ -3295,7 +3318,8 @@ ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
> int order, i;
> struct ext4_inode_info *ei = EXT4_I(ac->ac_inode);
> struct ext4_locality_group *lg;
> - struct ext4_prealloc_space *pa;
> + struct ext4_prealloc_space *pa, *cpa = NULL;
> + ext4_fsblk_t goal_block;
>
> /* only data can be preallocated */
> if (!(ac->ac_flags & EXT4_MB_HINT_DATA))
> @@ -3338,6 +3362,10 @@ ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
> /* The max size of hash table is PREALLOC_TB_SIZE */
> order = PREALLOC_TB_SIZE - 1;
>
> + goal_block = ac->ac_g_ex.fe_group * EXT4_BLOCKS_PER_GROUP(ac->ac_sb) +
> + ac->ac_g_ex.fe_start +
> + le32_to_cpu(EXT4_SB(ac->ac_sb)->s_es->s_first_data_block);
> +
> for (i = order; i < PREALLOC_TB_SIZE; i++) {
> rcu_read_lock();
> list_for_each_entry_rcu(pa, &lg->lg_prealloc_list[i],
> @@ -3345,17 +3373,19 @@ ext4_mb_use_preallocated(struct ext4_allocation_context *ac)
> spin_lock(&pa->pa_lock);
> if (pa->pa_deleted == 0 &&
> pa->pa_free >= ac->ac_o_ex.fe_len) {
> - atomic_inc(&pa->pa_count);
> - ext4_mb_use_group_pa(ac, pa);
> - spin_unlock(&pa->pa_lock);
> - ac->ac_criteria = 20;
> - rcu_read_unlock();
> - return 1;
> +
> + cpa = ext4_mb_check_group_pa(goal_block,
> + pa, cpa);
> }
cpa is initalized as NULL, and I could not see where we set cpa any
pointer before calling ext4_mb_check_group_pa(). If I understand
right, the code above passes a NULL pointer to
ext4_mb_check_group_pa(), which will result in just choose the pa
pointer directly, and bypass the distance calculation guided by the
goal block. Did I miss anything?
> spin_unlock(&pa->pa_lock);
> }
> rcu_read_unlock();
> }
> + if (cpa) {
> + ext4_mb_use_group_pa(ac, cpa);
> + ac->ac_criteria = 20;
> + return 1;
> + }
> return 0;
> }
>
> diff --git a/fs/inode.c b/fs/inode.c
> index b6726f6..d77f0ee 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -163,6 +163,7 @@ static struct inode *alloc_inode(struct super_block *sb)
> mapping->a_ops = &empty_aops;
> mapping->host = inode;
> mapping->flags = 0;
> + mapping->writeback_index = 0;
> mapping_set_gfp_mask(mapping, GFP_HIGHUSER_PAGECACHE);
> mapping->assoc_mapping = NULL;
> mapping->backing_dev_info = &default_backing_dev_info;
Could you explain what's this change for?
Regards,
Mingming
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