Jeff Moyer <jmoyer@xxxxxxxxxx> writes: > Mikulas Patocka <mpatocka@xxxxxxxxxx> writes: > >> Hi Jeff >> >> Thanks for testing. >> >> It would be interesting ... what happens if you take the patch 3, leave >> "struct percpu_rw_semaphore bd_block_size_semaphore" in "struct >> block_device", but remove any use of the semaphore from fs/block_dev.c? - >> will the performance be like unpatched kernel or like patch 3? It could be >> that the change in the alignment affects performance on your CPU too, just >> differently than on my CPU. > > It turns out to be exactly the same performance as with the 3rd patch > applied, so I guess it does have something to do with cache alignment. > Here is the patch (against vanilla) I ended up testing. Let me know if > I've botched it somehow. > > So, I next up I'll play similar tricks to what you did (padding struct > block_device in all kernels) to eliminate the differences due to > structure alignment and provide a clear picture of what the locking > effects are. After trying again with the same padding you used in the struct bdev_inode, I see no performance differences between any of the patches. I tried bumping up the number of threads to saturate the number of cpus on a single NUMA node on my hardware, but that resulted in lower IOPS to the device, and hence consumption of less CPU time. So, I believe my results to be inconclusive. After talking with Vivek about the problem, he had mentioned that it might be worth investigating whether bd_block_size could be protected using SRCU. I looked into it, and the one thing I couldn't reconcile is updating both the bd_block_size and the inode->i_blkbits at the same time. It would involve (afaiui) adding fields to both the inode and the block_device data structures and using rcu_assign_pointer and rcu_dereference to modify and access the fields, and both fields would need to protected by the same struct srcu_struct. I'm not sure whether that's a desirable approach. When I started to implement it, it got ugly pretty quickly. What do others think? For now, my preference is to get the full patch set in. I will continue to investigate the performance impact of the data structure size changes that I've been seeing. So, for the four patches: Acked-by: Jeff Moyer <jmoyer@xxxxxxxxxx> Jens, can you have a look at the patch set? We are seeing problem reports of this in the wild[1][2]. Cheers, Jeff [1] https://bugzilla.redhat.com/show_bug.cgi?id=824107 [2] https://bugzilla.redhat.com/show_bug.cgi?id=812129 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
