On Fri, May 21, 2021 at 12:33:52AM +0200, Jan Kara wrote: > Most of the merging happens at bio level. There should not be much > merging happening at request level anymore. Furthermore if we backmerged > a request to the previous one, the chances to be able to merge the > result to even previous request are slim - that could succeed only if > requests were inserted in 2 1 3 order. Merging more requests in Right, but some workload has this kind of pattern. For example of qemu IO emulation, it often can be thought as single job, native aio, direct io with high queue depth. IOs is originated from one VM, but may be from multiple jobs in the VM, so bio merge may not hit much because of IO emulation timing(virtio-scsi/blk's MQ, or IO can be interleaved from multiple jobs via the SQ transport), but request merge can really make a difference, see recent patch in the following link: https://lore.kernel.org/linux-block/3f61e939-d95a-1dd1-6870-e66795cfc1b1@xxxxxxx/T/#t > elv_attempt_insert_merge() will be difficult to handle when we want to > pass requests to free back to the caller of > blk_mq_sched_try_insert_merge(). So just remove the possibility of > merging multiple requests in elv_attempt_insert_merge(). This way will cause regression. Thanks, Ming
