content: Use at most 128MB of discardable memory on Android.

512MB is needed on desktop to not regress performance on
some important benchmarks but this limit is much more than
needed on Android. By reducing it to 128MB we can also
avoid using a different allocation size on Android. This
makes the discardable memory behavior more consistent
across platforms and improves the browser's ability to
purge memory on Android.

This change also reduces the amount of discardable memory
used on low end devices to 1/8th of the normal limit.

BUG=489174

Review URL: https://codereview.chromium.org/1013533002

Cr-Commit-Position: refs/heads/master@{#330754}

content/child/child_discardable_shared_memory_manager.cc

@@ -22,12 +22,7 @@ namespace content {
 namespace {
 
 // Default allocation size.
-#if defined(OS_ANDROID)
-// Larger allocation size on Android to avoid reaching the FD-limit.
-const size_t kAllocationSize = 32 * 1024 * 1024;
-#else
 const size_t kAllocationSize = 4 * 1024 * 1024;
-#endif
 
 // Global atomic to generate unique discardable shared memory IDs.
 base::StaticAtomicSequenceNumber g_next_discardable_shared_memory_id;

content/common/host_discardable_shared_memory_manager.cc

@@ -68,7 +68,12 @@ class DiscardableMemoryImpl : public base::DiscardableMemory {
 base::LazyInstance<HostDiscardableSharedMemoryManager>
     g_discardable_shared_memory_manager = LAZY_INSTANCE_INITIALIZER;
 
+#if defined(OS_ANDROID)
+// Limits the number of FDs used to 32, assuming a 4MB allocation size.
+const int64_t kMaxDefaultMemoryLimit = 128 * 1024 * 1024;
+#else
 const int64_t kMaxDefaultMemoryLimit = 512 * 1024 * 1024;
+#endif
 
 const int kEnforceMemoryPolicyDelayMs = 1000;
 
@@ -89,7 +94,11 @@ HostDiscardableSharedMemoryManager::HostDiscardableSharedMemoryManager()
     : memory_limit_(
           // Allow 25% of physical memory to be used for discardable memory.
           std::min(base::SysInfo::AmountOfPhysicalMemory() / 4,
-                   kMaxDefaultMemoryLimit)),
+                   base::SysInfo::IsLowEndDevice()
+                       ?
+                       // Use 1/8th of discardable memory on low-end devices.
+                       kMaxDefaultMemoryLimit / 8
+                       : kMaxDefaultMemoryLimit)),
       bytes_allocated_(0),
       memory_pressure_listener_(new base::MemoryPressureListener(
           base::Bind(&HostDiscardableSharedMemoryManager::OnMemoryPressure,