# Saving Simulation and Truncating Values in Memory

**URL:** <https://discourse.julialang.org/t/saving-simulation-and-truncating-values-in-memory/103095>\
**Category:** New to Julia\
**Created:** [August 22, 2023, 11:15pm UTC](https://discourse.julialang.org/t/saving-simulation-and-truncating-values-in-memory/103095 "2023-08-22T23:15:43Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![yoshi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yoshi/32/38259_2.png) [@yoshi](https://discourse.julialang.org/u/yoshi)\
**Post date:** [August 22, 2023, 11:15pm UTC](https://discourse.julialang.org/t/saving-simulation-and-truncating-values-in-memory/103095/1 "2023-08-22T23:15:43Z")

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I have a long simulation. I’m trying to find a way to save the values of my simulation to disk and truncate the values held in memory.

My guess is to use a callback. So far I have:

```julia
  using MAT

  function save_and_clear(u, t, integ)
        outFile = matopen("$(t)_X.mat","w");
          write(outFile,"u",u);
        close(outFile);

    end

    cb = FunctionCallingCallback(save_and_clear; funcat = [1000, 2000, 3000])

    @time sim = solve(prob, Tsit5(), maxiters=1e10, saveat=SaveTimes, reltol = reltolVal, callback = cb);

```

This is sort of working. I’m getting the values at `t = 1000, 2000, 3000`. What I want are all the values from t \in [0, 1000), t \in [1000, 2000) and t \in [2000, 3000). How I access those values? I was guessing they were in `integrator` but I can’t find them. Furthermore, once I save, t \in [0, 1000), how do I remove them from memory?

thanks!

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<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [August 23, 2023, 10:09am UTC](https://discourse.julialang.org/t/saving-simulation-and-truncating-values-in-memory/103095/2 "2023-08-23T10:09:37Z")

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If you instead do `FunctionCallingCallback(save_and_clear)`, it will use the default `func_everystep = true` and thus fire at every step, thus saving at every step.

If you also save `integ.k` you can also rebuild the interpolant if you wanted.

Note for low memory you want to do:

```julia
@time sim = solve(prob, Tsit5(), maxiters=1e10, save_on=false, reltol = reltolVal, callback = cb);

```

I.e. do all saving manually.

---

<div class="post-metadata">

**Author:** ![yoshi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yoshi/32/38259_2.png) [@yoshi](https://discourse.julialang.org/u/yoshi)\
**Post date:** [August 23, 2023, 9:50pm UTC](https://discourse.julialang.org/t/saving-simulation-and-truncating-values-in-memory/103095/3 "2023-08-23T21:50:20Z")

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In order to do this, I created two callback functions —

```julia
    function save_func(u,t,integ)
      return u     
    end
    saved_values = SavedValues(Float64, Matrix{Float64});
    collection_cb = SavingCallback(save_func, saved_values, saveat = SaveTimes);

    diskTimes = collect.([25000.0:25000.0:75000.0]);
    condition(u, t, integrator) = t ∈ diskTimes[1]
    function save_to_disk_and_clear_cb!(integ)
        outFile = matopen("$(integ.t)_X.mat","w");
          write(outFile,"tme",integ.opts.callback.discrete_callbacks[1].affect!.saved_values.t);
          #write(outFile,"V",integ.opts.callback.discrete_callbacks[1].affect!.saved_values.saveval[:,1]);
          #write(outFile,"conds",integ.opts.callback.discrete_callbacks[1].affect!.saved_values.saveval[:,14:20]);
          write(outFile,"sol",integ.opts.callback.discrete_callbacks[1].affect!.saved_values.saveval);
        close(outFile);
        empty!(integ.opts.callback.discrete_callbacks[1].affect!.saved_values.t)
        empty!(integ.opts.callback.discrete_callbacks[1].affect!.saved_values.saveval)
    end
    saveToDisk_cb = DiscreteCallback(condition, save_to_disk_and_clear_cb!)

    cbset = CallbackSet(collection_cb,saveToDisk_cb);

```

I did this while keeping `save_on = false` as chris suggested.

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<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [August 24, 2023, 5:43am UTC](https://discourse.julialang.org/t/saving-simulation-and-truncating-values-in-memory/103095/4 "2023-08-24T05:43:15Z")

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You could also have just done that with only a single FunctionCallingCallback

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<div class="post-metadata">

**Author:** ![yoshi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yoshi/32/38259_2.png) [@yoshi](https://discourse.julialang.org/u/yoshi)\
**Post date:** [August 25, 2023, 6:42pm UTC](https://discourse.julialang.org/t/saving-simulation-and-truncating-values-in-memory/103095/5 "2023-08-25T18:42:29Z")

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the second line should be `return copy(u)`