Mikulas Patocka <mpatocka@xxxxxxxxxx> writes: > On Tue, 25 Sep 2012, Jeff Moyer wrote: > >> 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. > > For me, the fourth patch with RCU-based locks performed better, so I am > submitting that. > >> 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? > > Using RCU doesn't seem sensible to me (except for lock implementation, as > it is in patch 4). The major problem is that the block layer reads > blocksize multiple times and when different values are read, a crash may > happen - RCU doesn't protect you against that - if you read a variable > multiple times in a RCU-protected section, you can still get different > results. SRCU is sleepable, so could be (I think) used in the same manner as your rw semaphore. The only difference is that it would require changing the bd_blocksize and the i_blkbits to pointers and protecting them both with the same srcu struct. Then, the inode i_blkbits would also need to be special cased, so that we only require such handling when it is associated with a block device. It got messy. > If we wanted to use RCU, we would have to read blocksize just once and > pass the value between all functions involved - that would result in a > massive code change. If we did that, we wouldn't need rcu at all, would we? Cheers, Jeff -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
