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From: Peter Xu <peterx@redhat.com>
To: kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	peterx@redhat.com, Maxim Levitsky <mlevitsk@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: [PATCH v3 5/7] KVM: X86: MMU: Tune PTE_LIST_EXT to be bigger
Date: Fri, 30 Jul 2021 18:04:53 -0400	[thread overview]
Message-ID: <20210730220455.26054-6-peterx@redhat.com> (raw)
In-Reply-To: <20210730220455.26054-1-peterx@redhat.com>

Currently rmap array element only contains 3 entries.  However for EPT=N there
could have a lot of guest pages that got tens of even hundreds of rmap entry.

A normal distribution of a 6G guest (even if idle) shows this with rmap count
statistics:

Rmap_Count:     0       1       2-3     4-7     8-15    16-31   32-63   64-127  128-255 256-511 512-1023
Level=4K:       3089171 49005   14016   1363    235     212     15      7       0       0       0
Level=2M:       5951    227     0       0       0       0       0       0       0       0       0
Level=1G:       32      0       0       0       0       0       0       0       0       0       0

If we do some more fork some pages will grow even larger rmap counts.

This patch makes PTE_LIST_EXT bigger so it'll be more efficient for the general
use case of EPT=N as we do list reference less and the loops over PTE_LIST_EXT
will be slightly more efficient; but still not too large so less waste when
array not full.

It should not affecting EPT=Y since EPT normally only has zero or one rmap
entry for each page, so no array is even allocated.

With a test case to fork 500 child and recycle them ("./rmap_fork 500" [1]),
this patch speeds up fork time of about 29%.

    Before: 473.90 (+-5.93%)
    After:  366.10 (+-4.94%)

[1] https://github.com/xzpeter/clibs/commit/825436f825453de2ea5aaee4bdb1c92281efe5b3

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 arch/x86/kvm/mmu/mmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 16c99f771c9e..c0b452bb5dd9 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -137,8 +137,8 @@ module_param(dbg, bool, 0644);
 
 #include <trace/events/kvm.h>
 
-/* make pte_list_desc fit well in cache line */
-#define PTE_LIST_EXT 3
+/* make pte_list_desc fit well in cache lines */
+#define PTE_LIST_EXT 15
 
 struct pte_list_desc {
 	u64 *sptes[PTE_LIST_EXT];
-- 
2.31.1


  parent reply	other threads:[~2021-07-30 22:05 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-30 22:04 [PATCH v3 0/7] KVM: X86: Some light optimizations on rmap logic Peter Xu
2021-07-30 22:04 ` [PATCH v3 1/7] KVM: Allow to have arch-specific per-vm debugfs files Peter Xu
2021-08-03 11:15   ` Greg KH
2021-08-03 19:25     ` Peter Xu
2021-07-30 22:04 ` [PATCH v3 2/7] KVM: X86: Introduce pte_list_count() helper Peter Xu
2021-07-30 22:04 ` [PATCH v3 3/7] KVM: X86: Introduce kvm_mmu_slot_lpages() helpers Peter Xu
2021-07-30 22:04 ` [PATCH v3 4/7] KVM: X86: Introduce mmu_rmaps_stat per-vm debugfs file Peter Xu
2021-08-02 15:25   ` Paolo Bonzini
2021-08-03 19:14     ` Peter Xu
2021-08-05 18:19     ` Sean Christopherson
2021-07-30 22:04 ` Peter Xu [this message]
2021-07-30 22:06 ` [PATCH v3 6/7] KVM: X86: Optimize pte_list_desc with per-array counter Peter Xu
2021-07-30 22:06 ` [PATCH v3 7/7] KVM: X86: Optimize zapping rmap Peter Xu

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