Add HPC FAQ [no ci]

Author: emstoudenmire

docs/settings.jl

@@ -50,6 +50,7 @@ settings = Dict(
       "ITensor Development FAQs" => "faq/Development.md",
       "Relationship of ITensor to other tensor libraries FAQs" => "faq/RelationshipToOtherLibraries.md",
       "Julia Package Manager FAQs" => "faq/JuliaPkg.md",
+      "High-Performance Computing FAQs" => "faq/HPC.md",
     ],
     "Upgrade guides" => ["Upgrading from 0.1 to 0.2" => "UpgradeGuide_0.1_to_0.2.md"],
     "ITensor indices and Einstein notation" => "Einsum.md",

docs/src/faq/HPC.md

@@ -0,0 +1,27 @@
+# High Performance Computing (HPC) Frequently Asked Questions
+
+## My code is using a lot of RAM - what can I do about this?
+
+Tensor network algorithms can often use a large amount of RAM. However, on top
+of this essential fact, the Julia programming languge is also "garbage collected"
+which means that unused memory isn't given back to the operating system right away, 
+but only on a schedule determined by the Julia runtime. In cases where you code
+allocates a lot of memory very quickly, this can lead to high memory usage.
+
+Fortunately, one simple step you can take to potentially help with this is to pass
+the `--heap-size-hint` flag to the Julia program when you start it. For example,
+you can call Julia as:
+```
+julia --heap-size-hint=100G
+```
+When you pass this heap size, Julia will try to keep the memory usage at or below this
+value if possible.
+
+In cases where this does not work, your code simply may be allocating too much memory.
+Be sure not to allocate over and over again inside of "hot" loops which execute many times.
+
+Another possibility is that you are simply working with a tensor network with large 
+bond dimensions, which may fundamentally use a lot of memory. In those cases, you can 
+try to use features such as "write to disk mode" of the ITensor DMRG code or other related
+techniques. (See the `write_when_maxdim_exceeds` keyword of the ITensor `dmrg` function.)
+