[PATCH 2/3] Add Lazy slab initialization

[Owen Smith <owen.smith@xxxxxxxxxx>]

Only initialize 1/4 of a slab initially, unless a reservation is defined.
Allocate and initialize each slab object once this initial allocation has been
exceeded.

This is to decrease the overall resource usage when each cache object's
constructor allocates additional memory.

Signed-off-by: Owen Smith <owen.smith@xxxxxxxxxx>
---
 src/xenbus/cache.c | 27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/src/xenbus/cache.c b/src/xenbus/cache.c
index 247f244..d695ee3 100644
--- a/src/xenbus/cache.c
+++ b/src/xenbus/cache.c
@@ -61,6 +61,7 @@ typedef struct _XENBUS_CACHE_SLAB {
     LIST_ENTRY      ListEntry;
     USHORT          MaximumOccupancy;
     USHORT          CurrentOccupancy;
+    USHORT          CurrentAllocated;
     ULONG           *Mask;
     UCHAR           Buffer[1];
 } XENBUS_CACHE_SLAB, *PXENBUS_CACHE_SLAB;
@@ -301,6 +302,7 @@ CacheCreateSlab(
     ULONG               Size;
     LONG                Index;
     LONG                SlabCount;
+    LONG                ObjectsToAllocate;
     NTSTATUS            status;
 
     NumberOfBytes = P2ROUNDUP(FIELD_OFFSET(XENBUS_CACHE_SLAB, Buffer) +
@@ -326,6 +328,7 @@ CacheCreateSlab(
     Slab->Magic = XENBUS_CACHE_SLAB_MAGIC;
     Slab->Cache = Cache;
     Slab->MaximumOccupancy = (USHORT)Count;
+    Slab->CurrentAllocated = 0;
 
     Size = P2ROUNDUP(Count, BITS_PER_ULONG);
     Size /= 8;
@@ -334,12 +337,18 @@ CacheCreateSlab(
     if (Slab->Mask == NULL)
         goto fail3;
 
-    for (Index = 0; Index < (LONG)Slab->MaximumOccupancy; Index++) {
+    ObjectsToAllocate = (Cache->Reservation == 0) ?
+                (LONG)Count / 4 :
+                min((LONG)Cache->Reservation, (LONG)Count);
+
+    for (Index = 0; Index < ObjectsToAllocate; Index++) {
         PVOID Object = (PVOID)&Slab->Buffer[Index * Cache->Size];
 
         status = __CacheCtor(Cache, Object);
         if (!NT_SUCCESS(status))
             goto fail4;
+
+        Slab->CurrentAllocated++;
     }
 
     CacheInsertSlab(Cache, Slab);
@@ -402,7 +411,7 @@ CacheDestroySlab(
     ASSERT(Cache->Cursor != &Cache->SlabList ||
            IsListEmpty(&Cache->SlabList));
 
-    Index = Slab->MaximumOccupancy;
+    Index = Slab->CurrentAllocated;
     while (--Index >= 0) {
         PVOID Object = (PVOID)&Slab->Buffer[Index * Cache->Size];
 
@@ -493,6 +502,20 @@ CacheGetObjectFromSlab(
     if (Slab->CurrentOccupancy == Slab->MaximumOccupancy)
         return NULL;
 
+    ASSERT3U(Slab->CurrentOccupancy, <=, Slab->CurrentAllocated);
+    ASSERT3U(Slab->CurrentAllocated, <=, Slab->MaximumOccupancy);
+    if (Slab->CurrentOccupancy == Slab->CurrentAllocated) {
+        NTSTATUS            status;
+
+        Object = (PVOID)&Slab->Buffer[Slab->CurrentAllocated * Cache->Size];
+
+        status = __CacheCtor(Cache, Object);
+        if (!NT_SUCCESS(status))
+            return NULL;
+
+        Slab->CurrentAllocated++;
+    }
+
     Index = __CacheMaskScan(Slab->Mask, Slab->MaximumOccupancy);
     BUG_ON(Index >= Slab->MaximumOccupancy);
 
-- 
2.31.1.windows.1