This patch series improves cr 0 and cr 1 passes of the allocator signficantly. Currently, at cr 0 and 1, we perform linear lookups to find the matching groups. That's very inefficient for large file systems where there are millions of block groups. At cr 0, we only care about the groups that have the largest free order >= the request's order and at cr 1 we only care about groups where average fragment size > the request size. so, this patchset introduces new data structures that allow us to perform cr 0 lookup in constant time and cr 1 lookup in log (number of groups) time instead of linear. For cr 0, we add a list for each order and all the groups are enqueued to the appropriate list based on the largest free order in its buddy bitmap. This allows us to lookup a match at cr 0 in constant time. For cr 1, we add a new rb tree of groups sorted by largest fragment size. This allows us to lookup a match for cr 1 in log (num groups) time. These optimizations can be enabled by passing "mb_optimize_scan" mount option. These changes may result in allocations to be spread across the block device. While that would not matter some block devices (such as flash) it may be a cause of concern for other block devices that benefit from storing related content togetther such as disk. However, it can be argued that in high fragmentation scenrio, especially for large disks, it's still worth optimizing the scanning since in such cases, we get cpu bound on group scanning instead of getting IO bound. Perhaps, in future, we could dynamically turn this new optimization on based on fragmentation levels for such devices. Verified that there are no regressions in smoke tests (-g quick -c 4k). Also, to demonstrate the effectiveness for the patch series, following experiment was performed: Created a highly fragmented disk of size 65TB. The disk had no contiguous 2M regions. Following command was run consecutively for 3 times: time dd if=/dev/urandom of=file bs=2M count=10 Here are the results with and without cr 0/1 optimizations: |---------+------------------------------+---------------------------| | | Without CR 0/1 Optimizations | With CR 0/1 Optimizations | |---------+------------------------------+---------------------------| | 1st run | 5m1.871s | 2m47.642s | | 2nd run | 2m28.390s | 0m0.611s | | 3rd run | 2m26.530s | 0m1.255s | |---------+------------------------------+---------------------------| The patch [3/6] "ext4: add mballoc stats proc file" is a modified version of the patch originally written by Artem Blagodarenko (artem.blagodarenko@xxxxxxxxx). With that patch, I ran following command with and without optimizations. dd if=/dev/zero of=/mnt/file bs=2M count=2 conv=fsync Without optimizations: useless_c0_loops: 3 useless_c1_loops: 39 useless_c2_loops: 0 useless_c3_loops: 0 With optimizations: useless_c0_loops: 0 useless_c1_loops: 0 useless_c2_loops: 0 useless_c3_loops: 0 This shows that CR0 and CR1 optimizations get rid of useless CR0 and CR1 loops altogether thereby significantly reducing the number of groups that get considered. Changes from V3: ---------------- - This feature is now enabled by default for disks which have at least 16 groups. This can be turned off by passing mb_optimize_scan=0 mount option and can be turned on unconditionally by passing mb_optimize_scan=1 mount option. - Reorganized stats by cr level - Added ability to send parsed options back to the caller in parse_options() functions in super.c Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@xxxxxxxxx> Harshad Shirwadkar (6): ext4: drop s_mb_bal_lock and convert protected fields to atomic ext4: add ability to return parsed options from parse_options ext4: add mballoc stats proc file ext4: add MB_NUM_ORDERS macro ext4: improve cr 0 / cr 1 group scanning ext4: add proc files to monitor new structures fs/ext4/ext4.h | 30 ++- fs/ext4/mballoc.c | 567 +++++++++++++++++++++++++++++++++++++++++++--- fs/ext4/mballoc.h | 22 +- fs/ext4/super.c | 78 +++++-- fs/ext4/sysfs.c | 6 + 5 files changed, 646 insertions(+), 57 deletions(-) -- 2.31.0.rc2.261.g7f71774620-goog
