Visualizing Haskell heap profiles in 2020

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Heap profiling is a feature of the Glasgow Haskell Compiler (GHC) that lets a program record its own memory usage by type, module, cost center, or other attribute, and write it to a program.hp file.

Here I review the existing tools—and introduce a new one—for visualizing and analyzing these profiles.

hp2ps

hp2ps is the standard heap profile visualizer, as it comes bundled with GHC.

Run it as

hp2ps -c benchmark.hp

(where -c makes the output colored), and it will produce the file benchmark.ps, which you can open with many document viewers.

Here’s what the output looks like:

An example graph produced by hp2ps

The example shows a heap profile by the cost center stack that allocated the data. As I mentioned, there are many other types of heap profiles, but this is what I’ll be using here as an example.

As you see, the cost centers on the right are truncated. I usually like to see them longer. They are actually truncated by the profiled program itself, not by the visualizer, so to get longer profiles, rerun your program with +RTS -hc -L500 to increase the maximum length from the default 25 to, say, 500.

However, hp2ps doesn’t deal well with long cost center stacks (or other long identifiers) by default: the whole page would be filled with identifiers, and there would be no room left for the graph itself. To work around that, pass -M to hp2ps. It produces a two-page .ps file, with the legend on the first page and the graph on the second one.

I found that viewers like Okular and Evince only display the second page of the two-page .ps file, but it works if you first convert the output to pdf with ps2pdf. Here’s what the output looks like:

Two-page output from hp2ps -M

hp2pretty

hp2pretty by Claude Heiland-Allen has a few advantages over hp2ps: a nicer output with transparency and grid lines, truncation of long cost center stacks, and the ability to write the full cost center stacks to a file using a --key option.

Run it simply as

hp2pretty benchmark.hp

and it will produce a file named benchmark.svg.

Example output of hp2pretty

Perl & R

Sometimes a quick look at the heap profile graph is all you need to understand what to do next. Other times, a more detailed analysis is required. In such cases, my favorite way is to convert an .hp file to csv and load it into R.

To convert an .hp file to csv, I wrote a short Perl script, hp2csv. (Unlike many tools written in Haskell, there’s a good chance it’ll continue working in 10 years.) Put it somewhere in your PATH, make it executable (chmod +x ~/bin/hp2csv), and run

hp2csv benchmark.hp > benchmark.csv

The CSV has a simple format:

time,name,value
0.094997,(487)getElements/CAF:getElements,40
0.094997,(415)CAF:$cfoldl'_r3hK,32
0.094997,(412)CAF:$ctoList_r3hH,32
0.094997,(482)match/main/Main.CAF,24
0.094997,(480)main/Main.CAF,32

where time is the time in seconds since the program start, name is the name of the cost center/type/etc. (depending on what kind of heap profiling you did), and value is the number of bytes.

Now let’s load this into R and try to reproduce the above graphs using ggplot.

library(tidyverse)
library(scales) # for a somewhat better color scheme

csv <- read_csv("benchmark.csv") %>%
  # convert bytes to megabytes
  mutate(value = value / 1e6) %>%
  # absent measurements are 0s
  complete(time,name, fill = list(value = 0))
  

# find top 15 entries and sort them
top_names <- csv %>%
  group_by(name) %>%
  summarize(sum_value = sum(value)) %>%
  arrange(desc(sum_value)) %>%
  head(n=15) %>%
  mutate(name_sorted = str_trunc(name,30),
         name_sorted = factor(name_sorted, levels=name_sorted))
top_entries <-
  inner_join(csv, top_names, by="name")

# Create a custom color palette based on the 'viridis' palette.
# Use 'sample' to shuffle the colors,
# so that adjacent areas are not similarly colored.
colors <- function(n) {
  set.seed(2020)
  sample(viridis_pal(option="A",alpha=0.7)(n))
}

theme_set(theme_bw())
ggplot(top_entries,aes(time,value,fill=name_sorted)) +
  geom_area(position="stack") +
  discrete_scale(aesthetics = "fill",
                 scale_name = "viridis modified",
                 palette = colors) +
  scale_y_continuous(breaks=function(limits) seq(0, floor(limits[[2]]), by=10)) +
  labs(x="seconds", y="MB", fill = "Cost center")
A stacked graph of the heap profile (produced with ggplot)

But these stacked plots are not always the best way to represent the data. Let’s see what happens if we try a simple line plot.

top_entries %>%
  ggplot(aes(time,value,color=name_sorted)) +
    geom_line() +
    scale_y_continuous(breaks=function(limits) seq(0, floor(limits[[2]]), by=5)) +
    labs(x="seconds", y="MB", color = "Cost center")
A line graph of the heap profile

This looks weird, doesn’t it? Do those lines merge, or does one of them just disappear?

To disentangle this graph a bit, we can add a random offset for each cost-center.

set.seed(2020)
top_entries %>%
  group_by(name) %>%
  mutate(value = value + runif(1,0,3)) %>%
  ungroup %>%
  ggplot(aes(time,value,color=name_sorted)) +
    geom_line() +
    scale_y_continuous(breaks=function(limits) seq(0, floor(limits[[2]]), by=5)) +
    labs(x="seconds", y="MB", color = "Cost center")
A line graph of the heap profile, with a random offset added per cost center

So it’s not a glitch, and indeed several cost centers have identical dynamics. It’s not hard to imagine why this could happen: think about tuples whose elements occupy the same amount of space but are produced by different cost centers. As these tuples are consumed and garbage-collected, the corresponding lines remain in perfect sync. But this effect wasn’t obvious at all from the stacked plot, was it?

Another thing that is hard to understand from a stacked plot is how different cost centers compare, say, in terms of their maximum resident size. But in R, we can easily visualize this with a simple bar plot:

top_entries <- csv %>%
  group_by(name) %>%
  summarize(max_value = max(value)) %>%
  filter(max_value >= 1) %>%
  arrange(max_value) %>%
  mutate(name = str_trunc(name, 120), name = factor(name, levels=name))
ggplot(top_entries, aes(name,max_value)) + geom_col(fill=viridis_pal(alpha=0.7)(5)[[4]]) +
  geom_text(aes(name,label=name),y=0,hjust="left") +
  labs(x="Cost center", y="Memory, MB") +
  scale_x_discrete(breaks=NULL) +
  scale_y_continuous(breaks=function(limits) seq(0, floor(limits[[2]]), by=5)) +
  coord_flip()
A bar plot of the maximum residenct size per cost center

Finally, in R you are not limited to just visualization; you can do all sorts of data analyses. For instance, a few years back I needed to verify that, in a server process, a certain function was not consuming increasingly more memory over time. I used this technique to load the heap profile into R and verify that with more confidence that I would have had from looking at a stacked graph.

Tools that didn’t make it: hp2any and hp/D3.js

One issue with big Haskell projects is that, if not actively maintained, they tend to bitrot due to the changes in the compiler, the Haskell dependencies or even the C dependencies.

One such example is Patai Gergely’s hp2any. It no longer builds with the current version of the network package because of some API changes. But even when I tried to build it with the included stack.yaml file, I got

glib          > Linking /tmp/stack214336/glib-0.13.6.0/.stack-work/dist/x86_64-linux-tinfo6/Cabal-2.2.0.1/setup/setup ...
glib          > Configuring glib-0.13.6.0...
glib          > build
glib          > Preprocessing library for glib-0.13.6.0..
glib          > setup: Error in C header file.
glib          >   
glib          > /usr/include/glib-2.0/glib/gspawn.h:76: (column 22) [FATAL] 
glib          >   >>> Syntax error!
glib          >   The symbol `__attribute__' does not fit here.
glib          >   

I’m guessing (only guessing) that this issue is fixed in the latest versions of the glib Haskell package, but we can’t benefit from that when using an old stack.yaml. This also shows a flaw in some people’s argument that if you put prospective upper bounds on your Haskell dependencies, your projects will build forever.

(At this point, someone will surely mention nix and how it would’ve helped here. It probably would, but as an owner of a 50GB /nix directory, I’m not so enthusiastic about adding another 5GB there consisting of old OpenGL and GTK libraries just to get a heap profile visualizer.)

Edward Z. Yang’s hp/D3.js no longer builds as it depends on hp2any-core. I also tried to use Edward’s hosted version at heap.ezyang.com, but it resulted in a 500 error.