[patch 11/15] ext2: preread inode backing blocks

Andrew Morton (akpm@zip.com.au)
Sun, 19 May 2002 12:43:36 -0700


When ext2 creates a new inode, perform an asynchronous preread against
its backing block.

Without this patch, many-file writeout gets stalled by having to read
many individual inode table blocks in the middle of writeback.

It's worth about a 20% gain in writeback bandwidth for the many-file
writeback case.

ext3 already reads the inode's backing block in
ext3_new_inode->ext3_mark_inode_dirty, so no change is needed there.

A backport to 2.4 would make sense.

=====================================

--- 2.5.16/fs/ext2/ialloc.c~tuning-20-ext2-preread-inode Sun May 19 11:49:49 2002
+++ 2.5.16-akpm/fs/ext2/ialloc.c Sun May 19 11:49:49 2002
@@ -218,6 +218,44 @@ error_return:
}

/*
+ * We perform asynchronous prereading of the new inode's inode block when
+ * we create the inode, in the expectation that the inode will be written
+ * back soon. There are two reasons:
+ *
+ * - When creating a large number of files, the async prereads will be
+ * nicely merged into large reads
+ * - When writing out a large number of inodes, we don't need to keep on
+ * stalling the writes while we read the inode block.
+ *
+ * FIXME: ext2_get_group_desc() needs to be simplified.
+ */
+static void ext2_preread_inode(struct inode *inode)
+{
+ unsigned long block_group;
+ unsigned long offset;
+ unsigned long block;
+ struct buffer_head *bh;
+ struct ext2_group_desc * gdp;
+
+ block_group = (inode->i_ino - 1) / EXT2_INODES_PER_GROUP(inode->i_sb);
+ gdp = ext2_get_group_desc(inode->i_sb, block_group, &bh);
+ if (gdp == NULL)
+ return;
+
+ /*
+ * Figure out the offset within the block group inode table
+ */
+ offset = ((inode->i_ino - 1) % EXT2_INODES_PER_GROUP(inode->i_sb)) *
+ EXT2_INODE_SIZE(inode->i_sb);
+ block = le32_to_cpu(gdp->bg_inode_table) +
+ (offset >> EXT2_BLOCK_SIZE_BITS(inode->i_sb));
+ bh = sb_getblk(inode->i_sb, block);
+ if (!buffer_uptodate(bh) && !buffer_locked(bh))
+ ll_rw_block(READA, 1, &bh);
+ __brelse(bh);
+}
+
+/*
* There are two policies for allocating an inode. If the new inode is
* a directory, then a forward search is made for a block group with both
* free space and a low directory-to-inode ratio; if that fails, then of
@@ -417,6 +455,7 @@ repeat:
return ERR_PTR(-EDQUOT);
}
ext2_debug ("allocating inode %lu\n", inode->i_ino);
+ ext2_preread_inode(inode);
return inode;

fail2:

-
-
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