Messages in this thread Patch in this message | | | Subject | [PATCH] mmap: avoid unnecessary anon_vma lock acquisition in vma_adjust() | From | Lee Schermerhorn <> | Date | Wed, 09 Sep 2009 16:10:46 -0400 |
| |
Against: 2.6.31-rc6
We noticed very erratic behavior [throughput] with the AIM7 shared workload running on recent distro [SLES11] and mainline kernels on an 8-socket, 32-core, 256GB x86_64 platform. On the SLES11 kernel [2.6.27.19+] with Barcelona processors, as we increased the load [10s of thousands of tasks], the throughput would vary between two "plateaus"--one at ~65K jobs per minute and one at ~130K jpm. The simple patch below causes the results to smooth out at the ~130k plateau.
But wait, there's more:
We do not see this behavior on smaller platforms--e.g., 4 socket/8 core. This could be the result of the larger number of cpus on the larger platform--a scalability issue--or it could be the result of the larger number of interconnect "hops" between some nodes in this platform and how the tasks for a given load end up distributed over the nodes' cpus and memories--a stochastic NUMA effect.
The variability in the results are less pronounced [on the same platform] with Shanghai processors and with mainline kernels. With 31-rc6 on Shanghai processors and 288 file systems on 288 fibre attached storage volumes, the curves [jpm vs load] are both quite flat with the patched kernel consistently producing ~3.9% better throughput [~80K jpm vs ~77K jpm] than the unpatched kernel.
Profiling indicated that the "slow" runs were incurring high[er] contention on an anon_vma lock in vma_adjust(), apparently called from the sbrk() system call.
The patch:
A comment in mm/mmap.c:vma_adjust() suggests that we don't really need the anon_vma lock when we're only adjusting the end of a vma, as is the case for brk(). The comment questions whether it's worth while to optimize for this case. Apparently, on the newer, larger x86_64 platforms, with interesting NUMA topologies, it is worth while--especially considering that the patch [if correct!] is quite simple.
We can detect this condition--no overlap with next vma--by noting a NULL "importer". The anon_vma pointer will also be NULL in this case, so simply avoid loading vma->anon_vma to avoid the lock. However, we apparently DO need to take the anon_vma lock when we're inserting a vma ['insert' non-NULL] even when we have no overlap [NULL "importer"], so we need to check for 'insert', as well.
I have tested with and without the 'file || ' test in the patch. This does not seem to matter for stability nor performance. I left this check/filter in, so we only optimize away the anon_vma lock acquisition when adjusting the end of a non- importing, non-inserting, anon vma.
If accepted, this patch may benefit the stable tree as well.
Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
mm/mmap.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
Index: linux-2.6.31-rc6/mm/mmap.c =================================================================== --- linux-2.6.31-rc6.orig/mm/mmap.c 2009-08-19 14:34:13.000000000 -0400 +++ linux-2.6.31-rc6/mm/mmap.c 2009-08-19 14:53:24.000000000 -0400 @@ -573,9 +573,9 @@ again: remove_next = 1 + (end > next-> /* * When changing only vma->vm_end, we don't really need - * anon_vma lock: but is that case worth optimizing out? + * anon_vma lock. */ - if (vma->anon_vma) + if ((file || insert || importer) && vma->anon_vma) anon_vma = vma->anon_vma; if (anon_vma) { spin_lock(&anon_vma->lock);
| |