Weird Cuda out of memory error when I decrease the input size

I was training my model using Yolo v3
When I set my input size = 416, I can train my model with batch size = 9 without any errors.
However, when I decrease my input size to 320, I ran into Cuda memory error even when my batch size = 7.
I found this particularly strange, has anyone encountered anything similar before?
Error happened when executing loss.backward()
Thank you in advance for helping me!

Ok this is getting even more strange.
Keeping my input size = 320
I changed my batch size from 9 to 16, the model started training flawlessly without any errors


Do you use cudnn? Do you use it in benchmark mode? What is the memory usage values when it actually run and what is the memory on your gpu?

Hi albanD

  1. No I did not explicitly use cudnn.
  2. I did not use it in benchmark mode.
  3. These were my memory and gpu memory usage when I ran my training with input size = 320, batch size = 9:

[2018-12-12 13:27:27,480] ram memory info before loss.backward(): svmem(total=17138393088, available=5950562304, percent=65.3, used=11187830784, free=5950562304)
[2018-12-12 13:27:27,481] max_memory_allocated before loss.backward(): 2788280832
[2018-12-12 13:27:27,481] memory_cached before loss.backward(): 2799042560
[2018-12-12 13:27:27,482] max_memory_cached before loss.backward(): 2799042560
[2018-12-12 13:27:27,482] memory_allocated before loss.backward(): 2772128768

Erros came right after this
line 70, in train
File “C:\Users\jayde\Anaconda3\envs\new_env\lib\site-packages\torch\”, line 93, in backward
torch.autograd.backward(self, gradient, retain_graph, create_graph)
File “C:\Users\jayde\Anaconda3\envs\new_env\lib\site-packages\torch\”, line 90, in backward
allow_unreachable=True) # allow_unreachable flag
RuntimeError: CUDA error: out of memory

However when I set input size = 320, batch size = 16, this is what I get:

[2018-12-12 13:32:11,560] ram memory info before loss.backward(): svmem(total=17138393088, available=5892562944, percent=65.6, used=11245830144, free=5892562944)
[2018-12-12 13:32:11,561] max_memory_allocated before loss.backward(): 4837138432
[2018-12-12 13:32:11,561] memory_cached before loss.backward(): 5020450816
[2018-12-12 13:32:11,562] max_memory_cached before loss.backward(): 5020450816
[2018-12-12 13:32:11,562] memory_allocated before loss.backward(): 4684070912
[2018-12-12 13:32:11,749] ram memory info after loss.backward(): svmem(total=17138393088, available=5946978304, percent=65.3, used=11191414784, free=5946978304)
[2018-12-12 13:32:11,749] max_memory_allocated after loss.backward(): 5460978176
[2018-12-12 13:32:11,749] memory_cached after loss.backward(): 6174408704
[2018-12-12 13:32:11,750] max_memory_cached after loss.backward(): 6174408704
[2018-12-12 13:32:11,750] memory_allocated after loss.backward(): 518663680
[2018-12-12 13:32:12,162] ram memory info before loss.backward(): svmem(total=17138393088, available=5980577792, percent=65.1, used=11157815296, free=5980577792)
[2018-12-12 13:32:12,162] max_memory_allocated before loss.backward(): 5460978176
[2018-12-12 13:32:12,163] memory_cached before loss.backward(): 6193283072
[2018-12-12 13:32:12,163] max_memory_cached before loss.backward(): 6193283072
[2018-12-12 13:32:12,163] memory_allocated before loss.backward(): 5199972864
[2018-12-12 13:32:12,315] ram memory info after loss.backward(): svmem(total=17138393088, available=5965262848, percent=65.2, used=111731

No memory errors were raised.

Thank you again for helping me!


Are you sharing the GPU with other people?
Pytorch reports <3GB used but cuda report 11+GB in use on the device ( in the case where it fails).
As you can see, in the case where it works with a bigger batch size, pytorch actually uses much more memory: 5/6GB.

If there is a lot of memory already used on the gpu even though nothing is running (you can check with nvidia-smi). You can use (sudo) lsof /dev/nvidiaX where X is the number of the gpu you want to check. This will list all process using the gpu more thoroughly than nvidia-smi. If there are some things that shouldn’t be here like old python process, you can get rid of them by killing them.

Hi @albanD
Thank you so much for your reply!

No I did not share GPU with anyone.
The 11+GB is my local ram but gpu memory. I’m using a 2080 so it has only 8 GB of vram.

The thing is it fails every single time if I set input size = 320, batch = 9. However if I do input size = 320, batch = 16 right after that, it runs perfectly every single time too.

I tried to clear cache before back propagation too but the error incurred .

Thank you again for helping me!


What happens if you add torch.backends.cudnn.enabled=False at the beginning of your code? Does the error still occurs?

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Hi @albanD

Yes it solves the problem!!
Thank you so much for helping!
On the other, I have a couple of question regarding to this.

  1. I have never set torch.backends.cudnn.enabled=True, so is True the default setting in pytorch?
  2. Why would setting it True resulting this error?
    3, In what occasion that setting torch.backends.cudnn.enabled=True would be beneficial?

So I did one more round of test,
if I set
torch.backends.cudnn.enabled = True
torch.backends.cudnn.benchmark = True
no errors were raised too.

But if I set
torch.backends.cudnn.benchmark = False
Then error appeared again.

Previously I was having this impression that the error was caused by the benchmark as it continues to search for the best algorithm. Now I am even more confused.

Once again thank you so much for your help!!!


Yes I just remembered, that there was a bug in some handling of cudnn algorithm.
Basically cudnn choose the fastest possible algorithm but that sometimes require more memory than what you have. This should be handled properly and select a less memory hangry algorithm. There was a bug in this fixed in this pr.
Not sure if it made it into 1.0 but I think it did. Enabling benchmark mode should fix the issue and keep the best runtime !

Thank you so much @albanD!

Just one last question, do we need to install cudnn separately or it will be automatically installed when conda install torch?

Thank you again for helping me!

It comes with the conda install and is enabled by default.

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