Re: [PATCH v3 01/17] ext4: add ext4_try_lock_group() to skip busy groups

[Ojaswin Mujoo]

On Mon, Jul 14, 2025 at 09:03:11PM +0800, Baokun Li wrote:
> When ext4 allocates blocks, we used to just go through the block groups
> one by one to find a good one. But when there are tons of block groups
> (like hundreds of thousands or even millions) and not many have free space
> (meaning they're mostly full), it takes a really long time to check them
> all, and performance gets bad. So, we added the "mb_optimize_scan" mount
> option (which is on by default now). It keeps track of some group lists,
> so when we need a free block, we can just grab a likely group from the
> right list. This saves time and makes block allocation much faster.
> 
> But when multiple processes or containers are doing similar things, like
> constantly allocating 8k blocks, they all try to use the same block group
> in the same list. Even just two processes doing this can cut the IOPS in
> half. For example, one container might do 300,000 IOPS, but if you run two
> at the same time, the total is only 150,000.
> 
> Since we can already look at block groups in a non-linear way, the first
> and last groups in the same list are basically the same for finding a block
> right now. Therefore, add an ext4_try_lock_group() helper function to skip
> the current group when it is locked by another process, thereby avoiding
> contention with other processes. This helps ext4 make better use of having
> multiple block groups.
> 
> Also, to make sure we don't skip all the groups that have free space
> when allocating blocks, we won't try to skip busy groups anymore when
> ac_criteria is CR_ANY_FREE.
> 
> Performance test data follows:
> 
> Test: Running will-it-scale/fallocate2 on CPU-bound containers.
> Observation: Average fallocate operations per container per second.
> 
> |CPU: Kunpeng 920   |          P80            |
> |Memory: 512GB      |-------------------------|
> |960GB SSD (0.5GB/s)| base  |    patched      |
> |-------------------|-------|-----------------|
> |mb_optimize_scan=0 | 2667  | 4821  (+80.7%)  |
> |mb_optimize_scan=1 | 2643  | 4784  (+81.0%)  |
> 
> |CPU: AMD 9654 * 2  |          P96            |
> |Memory: 1536GB     |-------------------------|
> |960GB SSD (1GB/s)  | base  |    patched      |
> |-------------------|-------|-----------------|
> |mb_optimize_scan=0 | 3450  | 15371 (+345%)   |
> |mb_optimize_scan=1 | 3209  | 6101  (+90.0%)  |
> 
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> Reviewed-by: Jan Kara <jack@xxxxxxx>

Hey Baokun, I reviewed some of the patches in v2 but i think that was
very last moment so I'll add the comments in this series, dont mind the
copy paste :)

The patch itself looks good, thanks for the changes.

Feel free to add:

 Reviewed-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>

Regards,
ojaswin