Bug 801818 - Add an API to check if the platform is a low-memory one. r=bsmedberg, blassey

xpcom/base/nsIMemory.idl

@@ -37,7 +37,7 @@
  * realloc() has failed.
  */
 
-[scriptable, uuid(59e7e77a-38e4-11d4-8cf5-0060b0fc14a3)]
+[scriptable, uuid(6aef11c4-8615-44a6-9711-98f43805693d)]
 interface nsIMemory : nsISupports
 {
     /**
@@ -93,5 +93,13 @@ interface nsIMemory : nsISupports
      * DEPRECATED - Always returns false.  See bug 592308.
      */
     boolean isLowMemory();
+
+    /**
+     * This predicate can be used to determine if the platform is a "low-memory"
+     * platform. Callers may use this to dynamically tune their behaviour
+     * to favour reduced memory usage at the expense of performance. The value
+     * returned by this function will not change over the lifetime of the process.
+     */
+    boolean isLowMemoryPlatform();
 };
 

xpcom/base/nsMemoryImpl.cpp

@@ -20,6 +20,11 @@
 #include "nsString.h"
 #include "mozilla/Services.h"
 
+#ifdef ANDROID
+#include <stdio.h>
+#define LOW_MEMORY_THRESHOLD_KB (256 * 1024)
+#endif
+
 static nsMemoryImpl sGlobalMemory;
 
 NS_IMPL_QUERY_INTERFACE1(nsMemoryImpl, nsIMemory)
@@ -56,6 +61,37 @@ nsMemoryImpl::IsLowMemory(bool *result)
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsMemoryImpl::IsLowMemoryPlatform(bool *result)
+{
+#ifdef ANDROID
+    static int sLowMemory = -1; // initialize to unknown, lazily evaluate to 0 or 1
+    if (sLowMemory == -1) {
+        sLowMemory = 0; // assume "not low memory" in case file operations fail
+        *result = false;
+
+        // check if MemTotal from /proc/meminfo is greater than LOW_MEMORY_THRESHOLD_KB
+        FILE* fd = fopen("/proc/meminfo", "r");
+        if (!fd) {
+            return NS_OK;
+        }
+        uint64_t mem = 0;
+        int rv = fscanf(fd, "MemTotal: %lu kB", &mem);
+        if (fclose(fd)) {
+            return NS_OK;
+        }
+        if (rv != 1) {
+            return NS_OK;
+        }
+        sLowMemory = (mem > LOW_MEMORY_THRESHOLD_KB) ? 0 : 1;
+    }
+    *result = (sLowMemory == 1);
+#else
+    *result = false;
+#endif
+    return NS_OK;
+}
+
 /*static*/ nsresult
 nsMemoryImpl::Create(nsISupports* outer, const nsIID& aIID, void **aResult)
 {

xpcom/tests/TestPipe.cpp

@@ -148,6 +148,11 @@ NS_IMETHODIMP BackwardsAllocator::IsLowMemory(bool* retval)
   return NS_OK;
 }
 
+NS_IMETHODIMP BackwardsAllocator::IsLowMemoryPlatform(bool* retval)
+{
+  *retval = false;
+  return NS_OK;
+}
 
 nsresult TestBackwardsAllocator()
 {