On Mon, Mar 7, 2011 at 6:08 AM, Ted Ts'o <tytso@xxxxxxx> wrote: > On Sun, Mar 06, 2011 at 10:15:41PM +0200, Amir Goldstein wrote: >> >> That sounds about right, but why do I need a new bit? >> Why can't I use EXT4_GROUP_INFO_NEED_INIT_BIT to tell me the exact >> same thing? > > The current meaning of NEED_INIT_BIT is that it indicates that the > group has been initialized once since the file system has been > mounted. It is used by ext4_mb_good_group() to know whether it can > rely on ext4_group_info->bb_free, ext4_group_info->bb_fragments, > ext4_group_info->bb_largest_free_order, et. al, without needing to > reload the buddy bitmap. > > We added this so that even if memory pressure has forced the buddy > bitmap and block allocation bitmaps out of memory, we have enough > information in the ext4_group_info summary array that we can quickly > decide whether or not a group is a likely good candidate to be > examined more closely to have the necessary free blocks. Without this > (relatively recent) change, the mballoc code might potentially need to > read in tens if not hundreds of block allocation bitmaps only to find > that it didn't have enough contiguous blocks, and then the memory > pressure would push the block bitmap out of memory again.... and file > system performance would go into the toilet. > Right... we need it. I also wanted to examine if clearing the NEED_INIT_BIT on add_group_blocks is really necessary. Couldn't the buddy bitmap of partial group be initialized with all blocks at the end "used", similar to the block bitmap itself? Then add_group_blocks() could just "free" the extra added blocks. > > So we don't want to disturb the meaning of this particular bit. If we > zap the NEED_INIT_BIT whenever we discover that the group's buddy > bitmap page has been pushed out of memory, then we will once again > need to read in massive numbers of block bitmaps because clearing the > bit effectively marks the summary information stored ext4_group_info > structure as invalid. > > - Ted > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
