refine GPU memory allocation policy (#6373)

* fix gpu memory allocation policy

* refine codes

* fix code style

* follow comments

paddle/memory/detail/system_allocator.cc

@@ -83,7 +83,7 @@ void* GPUAllocator::Alloc(size_t& index, size_t size) {
   paddle::platform::GpuMemoryUsage(available, capacity);
 
   // Reserve memory for page tables, etc.
-  size_t reserving = capacity - paddle::platform::GpuMaxAllocSize();
+  size_t reserving = 0.05 * capacity + paddle::platform::GpuMinChunkSize();
   size_t usable = available > reserving ? available - reserving : 0;
 
   // If remaining size no less than expected size, using general

paddle/memory/memory.cc

@@ -64,19 +64,21 @@ BuddyAllocator* GetGPUBuddyAllocator(int gpu_id) {
     int gpu_num = platform::GetCUDADeviceCount();
     as = new BuddyAllocator*[gpu_num];
     for (int gpu = 0; gpu < gpu_num; gpu++) {
-      platform::SetDeviceId(gpu);
-      as[gpu] = new BuddyAllocator(new detail::GPUAllocator,
+      as[gpu] = nullptr;
+    }
+  }
+  platform::SetDeviceId(gpu_id);
+  if (!as[gpu_id]) {
+    as[gpu_id] = new BuddyAllocator(new detail::GPUAllocator,
                                     platform::GpuMinChunkSize(),
                                     platform::GpuMaxChunkSize());
-    }
     VLOG(10) << "\n\nNOTE: each GPU device use "
              << FLAGS_fraction_of_gpu_memory_to_use * 100
              << "% of GPU memory.\n"
-             << "You can set environment variable '"
-             << platform::kEnvFractionGpuMemoryToUse
+             << "You can set GFlags environment variable '"
+             << "FLAGS_fraction_of_gpu_memory_to_use"
              << "' to change the fraction of GPU usage.\n\n";
   }
-  platform::SetDeviceId(gpu_id);
   return as[gpu_id];
 }