ex2: theory questions report

exercise2/Makefile

@@ -22,6 +22,15 @@ clean:
 show: $(OUTDIR)/mandel.bmp
 	feh $<
 
+pdf: report.md
+	pandoc report.md -o $(OUTDIR)/report.pdf --pdf-engine=typst
+
+zip: $(OUTDIR)/report.pdf Makefile mandel_mpi.c bench.py
+	zip $(OUTDIR)/handin.zip $^
+
+unzip: $(OUTDIR)/handin.zip
+	unzip $< -d $(OUTDIR)/handin
+
 # --- cpu-based ---
 $(OUT): $(SRC)
 	$(CC) $(CFLAGS) -o $(OUT) $<

exercise2/report.md

@@ -0,0 +1,56 @@
+---
+title: theory questions
+date: 2025-09-16
+author: fredrik robertsen
+---
+
+## 1) speed-up
+
+the **formula for speed-up** is given by
+$$\text{speed-up} = \frac{1}{\text{serialized-portion}}$$
+such that if a program has 20% serial, you will at most get a 5x speed-up by
+parallelizing.
+
+i did not get any speed-up. in fact, my program is blazingly slow: creating an
+image of twice the original XSIZE and YSIZE took closer to 5 seconds for my MPI
+program, whereas the provided CPU program did it in about 3 seconds. this is
+probably due to several issues, such as only running the MPI-program on my own
+laptop, thus likely not leveraging any parallel processing. thence and from
+other MPI overhead we obtain such discrepancies. i might have been able to break
+even, if i ran it on snotra. i just don't think my implementation is
+particularly fast, mostly due to the I/O part.
+
+## 2) weak- and strong scaling
+
+- **strong scaling** refers to scaling by increasing the amount of processors
+  while keeping the problem size fixed.
+- **weak scaling** refers to scaling by increasing both the problem size and
+  amount of processors so as to keep the amount of work per processor fixed.
+
+thought scenario from education-mode AI: adding one or more people to the task
+of washing 10 dishes and 100 dishes. both examples are strong scaling, because
+amount of dishes are the same, even if we add people. however, it highlights
+overhead. it might be harder to work together on 10 dishes as opposed to 100.
+then, if you were to hire people to wash dishes per 10 dishes you have to wash,
+then we are talking about weak scaling.
+
+the main difference is that strong scaling becomes applicable when you have
+a sufficiently big enough problem size to begin with, such that the coordination
+overhead becomes minimal. weak scaling works at any problem size, because it is
+scaled proportionally to the amount of processors.
+
+## 3) SIMD vs SPMD
+
+MPI is **SPMD**, single program multiple data.
+
+this is apparent from the fact that we have only a single program running,
+spawning subprocesses to handle the multiple data. program refers to how each
+subprocess has the same program code, using only clever if-statements to
+differentiate between relevant ranks.
+
+**SIMD** is different in that it uses instruction-level handling of multiple data
+through vectorization. i.e. it could perform arithmetic or calculations on many
+elements of an array simultaneously through a single instruction, rather than
+spinning up subprocesses for doing it in parallel. this requires hardware
+support through the cpu architecture, i.e. through vector registers. though
+i don't know much about this (yet), as we've focused on SPMD so far.

flake.nix

@@ -27,6 +27,8 @@
           ffmpeg
           mpi
           python3
+          pandoc
+          bc
         ];
         shellHook = ''
           echo welcome!