On Fri, 31 Aug 2012, Jeff Moyer wrote:
> Mikulas Patocka <mpatocka@xxxxxxxxxx> writes:
>
> > On Fri, 31 Aug 2012, Mikulas Patocka wrote:
> >
> >> Hi
> >>
> >> This is a series of patches to prevent a crash when when someone is
> >> reading block device and block size is changed simultaneously. (the crash
> >> is already happening in the production environment)
> >>
> >> The first patch adds a rw-lock to struct block_device, but doesn't use the
> >> lock anywhere. The reason why I submit this as a separate patch is that on
> >> my computer adding an unused field to this structure affects performance
> >> much more than any locking changes.
> >>
> >> The second patch uses the rw-lock. The lock is locked for read when doing
> >> I/O on the block device and it is locked for write when changing block
> >> size.
> >>
> >> The third patch converts the rw-lock to a percpu rw-lock for better
> >> performance, to avoid cache line bouncing.
> >>
> >> The fourth patch is an alternate percpu rw-lock implementation using RCU
> >> by Eric Dumazet. It avoids any atomic instruction in the hot path.
> >>
> >> Mikulas
> >
> > I tested performance of patches. I created 4GB ramdisk, I initially filled
> > it with zeros (so that ramdisk allocation-on-demand doesn't affect the
> > results).
> >
> > I ran fio to perform 8 concurrent accesses on 8 core machine (two
> > Barcelona Opterons):
> > time fio --rw=randrw --size=4G --bs=512 --filename=/dev/ram0 --direct=1
> > --name=job1 --name=job2 --name=job3 --name=job4 --name=job5 --name=job6
> > --name=job7 --name=job8
> >
> > The results actually show that the size of struct block_device and
> > alignment of subsequent fields in struct inode have far more effect on
> > result that the type of locking used. (struct inode is placed just after
> > struct block_device in "struct bdev_inode" in fs/block-dev.c)
> >
> > plain kernel 3.5.3: 57.9s
> > patch 1: 43.4s
> > patches 1,2: 43.7s
> > patches 1,2,3: 38.5s
> > patches 1,2,3,4: 58.6s
> >
> > You can see that patch 1 improves the time by 14.5 seconds, but all that
> > patch 1 does is adding an unused structure field.
> >
> > Patch 3 is 4.9 seconds faster than patch 1, althogh patch 1 does no
> > locking at all and patch 3 does per-cpu locking. So, the reason for the
> > speedup is different sizeof of struct block_device (and subsequently,
> > different alignment of struct inode), rather than locking improvement.
>
> How many runs did you do? Did you see much run to run variation?
These results come from two runs (which differed by no more than 1s), but
I observed the same phenomenon - difference in time due to the size of
block_device - many times before when I was doing benchmarking when
developing these patches.
I actually had to apply something like this to make the results not depend
on the size of block_dev.
I would be interested if the same difference could be observed on other
processors or if it is something specific to AMD K10 architecture.
---
fs/block_dev.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
Index: linux-3.5.3-fast/fs/block_dev.c
===================================================================
--- linux-3.5.3-fast.orig/fs/block_dev.c 2012-08-31 22:30:07.000000000 +0200
+++ linux-3.5.3-fast/fs/block_dev.c 2012-08-31 22:30:43.000000000 +0200
@@ -31,7 +31,10 @@
#include "internal.h"
struct bdev_inode {
- struct block_device bdev;
+ union {
+ struct block_device bdev;
+ char pad[0x140];
+ };
struct inode vfs_inode;
};
> > I would be interested if other people did performance testing of the
> > patches too.
>
> I'll do some testing next week, but don't expect to get to it before
> Wednesday.
>
> Cheers,
> Jeff
Mikulas
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