On 06/15/2010 04:55 PM, Dave Hansen wrote:
Note: this is the real meat of the patch set. It can be applied up
to this point, and everything will probably be improved, at least
a bit.
Of slab shrinkers, the VM code says:
* Note that 'shrink' will be passed nr_to_scan == 0 when the VM is
* querying the cache size, so a fastpath for that case is appropriate.
and it *means* it. Look at how it calls the shrinkers:
nr_before = (*shrinker->shrink)(0, gfp_mask);
shrink_ret = (*shrinker->shrink)(this_scan, gfp_mask);
So, if you do anything stupid in your shrinker, the VM will doubly
punish you.
Ouch.
The mmu_shrink() function takes the global kvm_lock, then acquires
every VM's kvm->mmu_lock in sequence. If we have 100 VMs, then
we're going to take 101 locks. We do it twice, so each call takes
202 locks. If we're under memory pressure, we can have each cpu
trying to do this. It can get really hairy, and we've seen lock
spinning in mmu_shrink() be the dominant entry in profiles.
This is guaranteed to optimize at least half of those lock
aquisitions away. It removes the need to take any of the locks
when simply trying to count objects.
Signed-off-by: Dave Hansen<dave@xxxxxxxxxxxxxxxxxx>
---
linux-2.6.git-dave/arch/x86/kvm/mmu.c | 33 +++++++++++++++++++++++----------
1 file changed, 23 insertions(+), 10 deletions(-)
diff -puN arch/x86/kvm/mmu.c~make_global_used_value arch/x86/kvm/mmu.c
--- linux-2.6.git/arch/x86/kvm/mmu.c~make_global_used_value 2010-06-09 15:14:30.000000000 -0700
+++ linux-2.6.git-dave/arch/x86/kvm/mmu.c 2010-06-09 15:14:30.000000000 -0700
@@ -891,6 +891,19 @@ static int is_empty_shadow_page(u64 *spt
}
#endif
+/*
+ * This value is the sum of all of the kvm instances's
+ * kvm->arch.n_used_mmu_pages values. We need a global,
+ * aggregate version in order to make the slab shrinker
+ * faster
+ */
+static unsigned int kvm_total_used_mmu_pages;
The variable needs to be at the top of the file.
+static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, int nr)
+{
+ kvm->arch.n_used_mmu_pages += nr;
+ kvm_total_used_mmu_pages += nr;
Needs an atomic operation, since there's no global lock here. To avoid
bouncing this cacheline around, make the variable percpu and make
readers take a sum across all cpus. Side benefit is that you no longer
need an atomic but a local_t, which is considerably cheaper.
+}
+
--
error compiling committee.c: too many arguments to function
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