hi,
With inclusion of Ritesh's inode lock scalability patches[1], the traditional performance reasons for dioread_nolock --- namely, removing the need to take an exclusive lock for Direct I/O read operations --- has been removed. [1] https://lore.kernel.org/r/20191212055557.11151-1-riteshh@xxxxxxxxxxxxx So... is it time to remove the code which supports dioread_nolock? Doing so would simplify the code base, and reduce the test matrix. This would also make it easier to restructure the write path when allocating blocks so that the extent tree is updated after writing out the data blocks, by clearing away the underbrush of dioread nolock first. If we do this, we'd leave the dioread_nolock mount option for backwards compatibility, but it would be a no-op and not actually do anything. Any objections before I look into ripping out dioread_nolock? The one possible concern that I considered was for Alibaba, which was doing something interesting with dioread_nolock plus nodelalloc. But looking at Liu Bo's explanation[2], I believe that their workload would be satisfied simply by using the standard ext4 mount options (that is, the default mode has the performance benefits when doing parallel DIO reads, and so the need for nodelalloc to mitigate the tail latency concerns which Alibaba was seeing in their workload would not be needed). Could Liu or someone from Alibaba confirm, perhaps with some benchmarks using their workload?
Currently we don't use dioread_nolock & nodelalloc in our internal servers, and we use dioread_nolock & delalloc widely, it works well. The initial reason we use dioread_nolock is that it'll also allocate unwritten extents for buffered write, and normally the corresponding inode won't be added to jbd2 transaction's t_inode_list, so while commiting transaction, it won't flush inodes' dirty pages, then transaction will commit quickly, otherwise in extream case, the time taking to flush dirty inodes will be very big, especially cgroup writeback is enabled. A previous discussion: https://marc.info/?l=linux-fsdevel&m=151799957104768&w=2 I think this semantics hidden behind diread_nolock is also important, so if planning to remove this mount option, we should keep same semantics. Regards, Xiaoguang Wang
[2] https://lore.kernel.org/linux-ext4/20181121013035.ab4xp7evjyschecy@US-160370MP2.local/ - Ted
