single flag to disable tlb shootdowns (for experimental purposes).

with tlb shootdowns disabled, radix scales linearly, and crange has sub-linear scaling. with VM_RADIX: $ time mapbench 1 12834002141 cycles $ time mapbench 2 12230873801 cycles $ time mapbench 16 12300269260 cycles with VM_CRANGE: $ time mapbench 1 12969300915 cycles $ time mapbench 2 15277712346 cycles $ time mapbench 16 58582887683 cycles

single flag to disable tlb shootdowns (for experimental purposes).
9999dcb5 · Nickolai Zeldovich · 323d260b · 9999dcb5
--- a/kernel/vm.cc
+++ b/kernel/vm.cc
@@ -18,6 +18,7 @@
 #include "kmtrace.hh"
 enum { vm_debug = 0 };
+enum { tlb_shootdown = 1 };
 enum { tlb_lazy = 1 };
 /*
@@ -320,8 +321,14 @@ vmap::copy(int share)
 #endif
  }
-  if (share)
+  if (share) {
-    tlbflush();  // Reload hardware page table
+    // Reload hardware page table
+    if (tlb_shootdown) {
+      tlbflush();
+    } else {
+      lcr3(rcr3());
+    }
+  }
  nm->brk_ = brk_;
@@ -434,11 +441,13 @@ again:
          }
        }
      });
+  if (tlb_shootdown) {
    if (needtlb && dotlb)
      tlbflush();
    else
      if (tlb_lazy)
        tlbflush(myproc()->unmap_tlbreq_);
+  }
  return vma_start;
 }
@@ -488,7 +497,7 @@ vmap::remove(uptr vma_start, uptr len)
        }
      }
    });
-  if (needtlb) {
+  if (tlb_shootdown && needtlb) {
    if (tlb_lazy) {
      myproc()->unmap_tlbreq_ = tlbflush_req++;
    } else {
@@ -578,7 +587,11 @@ vmap::pagefault(uptr va, u32 err)
    if (pagefault_wcow(m) < 0)
      return -1;
+    if (tlb_shootdown) {
      tlbflush();
+    } else {
+      lcr3(rcr3());
+    }
    goto retry;
  }