Baokun Li <libaokun1@xxxxxxxxxx> writes: > 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. It seems this makes block allocation non deterministic, but depend on the system load. I can see where this could cause problems when reproducing bugs at least, but perhaps also in other cases. Better perhaps just round robin the groups? Or at least add a way to turn it off. -Andi
