PyTorch Lightning DDP crashes with unused parameters

[atnikos]

Bug description

When I try to run my code with PL2.0 and Pytorch2.0 using DDP strategy on 2 GPUs I receive the following error which did not happen with previous versions.

    return forward_call(*args, **kwargs)
  File "/home/nathanasiou/.venvs/space_updated/lib/python3.10/site-packages/torch/nn/parallel/distributed.py", line 1139, in forward
    if torch.is_grad_enabled() and self.reducer._rebuild_buckets():
RuntimeError: It looks like your LightningModule has parameters that were not used in producing the loss returned by training_step. If this is intentional, you must enable the detection of unused parameters in DDP, either by setting the string value `strategy='ddp_find_unused_parameters_true'` or by setting the flag in the strategy with `strategy=DDPStrategy(find_unused_parameters=True)`.

Once I change the strategy to strategy=ddp_find_unused_parameters_true the code turns terribly slow i.e. 12 hours each epochs while it was taking 2 minutes or less before.

How can I deal with this? I want some of my parameters to stay frozen and also want to be able to train using DDP and get the speed advantages. Is there a way I can do something with these parameters and switch off find unused parameters flag?

How to reproduce the bug

No response

Error messages and logs

# Error messages and logs here please

Environment

Current environment

PyTorch version: 2.0.0+cu117
Is debug build: False
CUDA used to build PyTorch: 11.7
ROCM used to build PyTorch: N/A

OS: Ubuntu 20.04.5 LTS (x86_64)
GCC version: (Ubuntu 9.4.0-1ubuntu1~20.04.1) 9.4.0
Clang version: Could not collect
CMake version: version 3.26.0
Libc version: glibc-2.31

Python version: 3.10.10 (main, Feb  8 2023, 14:50:01) [GCC 9.4.0] (64-bit runtime)
Python platform: Linux-5.15.0-67-generic-x86_64-with-glibc2.31
Is CUDA available: True
CUDA runtime version: 11.7.64
CUDA_MODULE_LOADING set to: LAZY
GPU models and configuration: GPU 0: Quadro RTX 5000
Nvidia driver version: 525.89.02
cuDNN version: Could not collect
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

CPU:
Architecture:                    x86_64
CPU op-mode(s):                  32-bit, 64-bit
Byte Order:                      Little Endian
Address sizes:                   46 bits physical, 48 bits virtual
CPU(s):                          12
On-line CPU(s) list:             0-11
Thread(s) per core:              2
Core(s) per socket:              6
Socket(s):                       1
NUMA node(s):                    1
Vendor ID:                       GenuineIntel
CPU family:                      6
Model:                           85
Model name:                      Intel(R) Xeon(R) W-2133 CPU @ 3.60GHz
Stepping:                        4
CPU MHz:                         3600.000
CPU max MHz:                     3900,0000
CPU min MHz:                     1200,0000
BogoMIPS:                        7200.00
Virtualization:                  VT-x
L1d cache:                       192 KiB
L1i cache:                       192 KiB
L2 cache:                        6 MiB
L3 cache:                        8,3 MiB
NUMA node0 CPU(s):               0-11
Vulnerability Itlb multihit:     KVM: Mitigation: VMX disabled
Vulnerability L1tf:              Mitigation; PTE Inversion; VMX conditional cache flushes, SMT vulnerable
Vulnerability Mds:               Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Meltdown:          Mitigation; PTI
Vulnerability Mmio stale data:   Mitigation; Clear CPU buffers; SMT vulnerable
Vulnerability Retbleed:          Mitigation; IBRS
Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl and seccomp
Vulnerability Spectre v1:        Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2:        Mitigation; IBRS, IBPB conditional, RSB filling, PBRSB-eIBRS Not affected
Vulnerability Srbds:             Not affected
Vulnerability Tsx async abort:   Mitigation; Clear CPU buffers; SMT vulnerable
Flags:                           fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_l3 invpcid_single pti intel_ppin ssbd mba ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm cqm mpx rdt_a avx512f avx512dq rdseed adx smap clflushopt clwb intel_pt avx512cd avx512bw avx512vl xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local dtherm ida arat pln pts hwp hwp_act_window hwp_epp hwp_pkg_req md_clear flush_l1d arch_capabilities

Versions of relevant libraries:
[pip3] numpy==1.22.3
[pip3] pytorch-lightning==2.0.0
[pip3] torch==2.0.0
[pip3] torchaudio==2.0.1
[pip3] torchmetrics==0.11.4
[pip3] torchvision==0.15.1
[pip3] triton==2.0.0
[conda] No relevant packages

More info

No response

[carmocca]

2.0 with ddp_find_unused_parameters_true should match the speed of <2.0 with ddp. Otherwise there might be a bug somewhere

[delta-func]

I think I am having the same issue. Although setting ddp_find_unused_parameters_true is not that slow in my case.

I double-checked with the following code and printed nothing in between enter and exit.

def on_before_optimizer_step(self, optimizer) -> None:
    print("on_before_opt enter")
    for p in self.trainable_params:
        if p.grad is None:
            print(p)
    print("on_before_opt exit")

[awaelchli]

on_before_optimizer_step is not the right place to check grads, because the training_step and backward runs within the optimizer closure. You can inspect your grads in on_after_backward().

The limitation of DDP cannot be overcome. Your forward/backward needs to either use all parameters or we need to allow DDP to "find" which parameters are unused. Lightning 2.0 simply switched the default. You can enable static graph
DDPStrategy(find_unused_parameters=True, static_graph=True) to see if you get a speed up.

If there is no clear evidence of a bug, we would close the issue. Let us know how it goes.

[delta-func]

Hi thanks for the reply. I tried on_after_backward(), but I got the same result. Nothing prints in between.

Maybe this is to do with my usage of state_dict_hook which deletes some of the parameters to exclude from checkpointing. Although theoretically I think it should not affect DDP. However If you think this is the cause, I will file it as a separate issue.

def on_after_backward(self) -> None:
    print("on_after_backward enter")
    for p in self.trainable_params:
        if p.grad is None:
            print(p)
    print("on_after_backward exit")

[zyangH1]

I meet the same problem.

It outputs the names of the parameters that I need to freeze.

[zyangH1]

Do I have to set find_unused_parameters = True ？

[atnikos]

@zyangH1 Yes, you have to do it if you have unused parameters.
@awaelchli I am not getting any speedups using 2 devices compared to a single one when completing an epoch i.e. same time to complete an epoch for 1 device with batch size 32 and 2 devices with batch size 16 is this expected as well? I don't find a reason why this happens (previously it was faster)

[awaelchli]

same time to complete an epoch for 1 device with batch size 32 and 2 devices with batch size 16 is this expected as well?

Yes of course that's expected. The single GPU experiment will probably be a bit faster even, since DDP has some communication overhead.

[meghana-kshirsagar]

I have the same issue. I am trying to finetune/retrain a model (ESM) and my initial code was using DDP, where I saw this error. I rewrote my code using pytorch lightning DDP and the error persists. It is very non-deterministic, sometimes an epoch finishes while sometimes it does not.

There are no unused parameters - I tried both find_unused_parameters=True (where it reports: Warning: find_unused_parameters=True was specified in DDP constructor, but did not find any unused parameters in the forward pass) and also the more explicit function as @delta-func proposed. In both cases, there are no unused parameters, yet I get the error:

RuntimeError: It looks like your LightningModule has parameters that were not used in producing the loss returned by training_step.
RuntimeError: Rank 2 successfully reached monitoredBarrier, but received errors while waiting for send/recv from rank 0.

Could this be related to disk I/O? Some posts suggested changing num_workers in the dataloader.

Upon setting static_graph=True, I still get the below error

RuntimeError: It looks like your LightningModule has parameters that were not used in producing the loss returned by training_step. If this is intentional, you must enable the detection of unused parameters in DDP, either by setting the string value strategy='ddp_find_unused_parameters_true' or by setting the flag in the strategy with `strategy=DDPStrategy(find_unused_parameters=True)