# Pytorch - store weight updates for momemntum

I am trying to implement momentum in my implementation of SGD with momentum. From my understanding this update look like this:

``````parameters -= (lr * (p.grad*0.1 + p_delta_prev*0.9))
``````

My question is how I should store my previous deltas from every update

here is what I have in my update function:

``````#we now want to do the update with momentum
#momentum takes derivative, multiplies it by 0.1, then takes the previous update,
#multiplies it by 0.9 and we add the two together
#alpha = 0.1, beta = 0.9;  p-=grad*0.1 + p*0.9
def update(x,y,lr):
wd = 1e-5
y_hat = model(x)
# weight decay
w2 = 0.
for p in model.parameters(): w2 += (p**2).sum()
loss = loss_func(y_hat, y) + w2*wd
loss.backward()
for p in model.parameters():
#p.grad is the slope of the line of that parameter
#current_p-previous_p to get difference
p_update = (lr * (p.grad*0.1 + p*0.9))
p.sub_(p_update)
return loss.item()
``````

Here the `p*0.9` should be replace by the p_delta_prev. But how should I store these deltas for every parameter? If I save them to a tensor wouldn't I would be effectively copying the weight deltas to memory making my model two times the size. What would be a good way to accomplish this? I do not want to use an inbuilt function that does that activation for me. I did look into the pytorch sgd.py and it looks like the store the states.

EDIT: I have updated the code:

``````#we now want to do the update with momentum
#momentum takes derivative, multiplys it by 0.1, then takes the previous update,
#multiplies it by 0.9 and we add the two together
#alpha = 0.1, beta = 0.9;  p-=grad*0.1 + p*0.9
p_delta = {}
def update(x,y,lr):
wd = 1e-5
y_hat = model(x)
# weight decay
w2 = 0.
for p in model.parameters(): w2 += (p**2).sum()
loss = loss_func(y_hat, y) + w2*wd
loss.backward()
i = 0
for p in model.parameters():
#p.grad is the slope of the line of that parameter
if i not in p_delta:#check if key exists
p_delta[i] = torch.zeros_like(p)
p_update = (lr *p.grad) + (p_delta[i]*0.9)
p_delta[i] = p_update.clone()
p.sub_(p_update)
I think the code in the excel spreadsheet is incorrect. Jeremy seems to show: `lr* ((p.grad*0.1) + (p_delta[i]*0.9))` but many tutorials seem to show: `(lr *p.grad) + (p_delta[i]*0.9)` If we implement Jeremy’s code the loss actually is slower than vanilla GD. The part of the video is here: https://youtu.be/CJKnDu2dxOE?t=6581