ext4: improve cr 0 and cr 1 passes 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 [2/5] "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: mballoc: reqs: 41 success: 1 groups_scanned: 63 groups_considered: 20643620 extents_scanned: 7851 goal_hits: 0 2^n_hits: 1 breaks: 39 lost: 0 useless_c0_loops: 3 useless_c1_loops: 39 useless_c2_loops: 0 useless_c3_loops: 0 buddies_generated: 491561/491520 buddies_time_used: 13078539152 preallocated: 0 discarded: 0 With optimizations: mballoc: reqs: 42 success: 1 groups_scanned: 62 groups_considered: 1011 extents_scanned: 8062 goal_hits: 0 2^n_hits: 0 breaks: 40 lost: 0 useless_c0_loops: 0 useless_c1_loops: 0 useless_c2_loops: 0 useless_c3_loops: 0 buddies_generated: 491561/491520 buddies_time_used: 13165943648 preallocated: 0 discarded: 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 V2: ---------------- - Added mb_linear_limit sysfs tunable that controls how many groups should the allocator search in linear fashion before consulting the the new data structures. - Added following optimizations: * Full groups are not added to either structures * MB_OPTIMIZE_SCAN is disabled for small file systems - Updated documentation in the code - Made output of mb_structs_summary output file to be YAML compatible - Small fixes to change location of increment ac_groups_considered variable and added missed MB_NUM_ORDERS macro Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@xxxxxxxxx> Harshad Shirwadkar (5): ext4: drop s_mb_bal_lock and convert protected fields to atomic 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 | 24 +- fs/ext4/mballoc.c | 541 +++++++++++++++++++++++++++++++++++++++++++--- fs/ext4/mballoc.h | 20 ++ fs/ext4/super.c | 6 +- fs/ext4/sysfs.c | 6 + 5 files changed, 564 insertions(+), 33 deletions(-) -- 2.30.1.766.gb4fecdf3b7-goog
