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Performance regression for hash_insert when using bash's internal malloc


From: Eduardo A . Bustamante López
Subject: Performance regression for hash_insert when using bash's internal malloc
Date: Sat, 22 Nov 2014 16:50:24 -0600
User-agent: Mutt/1.5.23 (2014-03-12)

I noticed that bash from the devel branch was taking too much time to start
when invoked as an interactive shell. I happen to have bash-completion
installed.

See this:

| address@hidden:~$ for sh in /bin/bash /tmp/bash/devel-O2/bin/bash; do time 
"$sh" -i <<< ''; done
| address@hidden:~$ 
| address@hidden:~$ exit
| 
| real    0m0.071s
| user    0m0.060s
| sys 0m0.004s
| address@hidden:~$ 
| address@hidden:~$ exit
| 
| real    0m2.497s
| user    0m2.484s
| sys 0m0.012s


>From 0.071s to 2.497s, that's a lot of time...

I managed to track the issue to the calls to 'complete ... NAME', which do a
hash_insert for each name (bash-completion does a lot of 'complete' calls).

So, in order to try and isolate the issue, I wrote the following test script
(the relevant part is highlighted):



address@hidden ~ % cat test
#!/bin/bash

: <<'REQUIRES'
* moreutils (for 'ts')
* gnuplot
REQUIRES

unset tmpdir tmpimg
trap 'rm -rf "$tmpdir"' EXIT

tmpdir=$(mktemp -d)
tmpimg=$(mktemp)

shells=(
/tmp/bash/master-O2/bin/bash
/tmp/bash/4.3-rc2-O2/bin/bash
/tmp/bash/devel-O2/bin/bash
/tmp/bash/devel-nobashmalloc-O2/bin/bash
)

cd "$tmpdir" || exit 1

for i in "address@hidden"; do
    sh=${shells[i]}
    for _ in {1..20}; do
    "$sh" -c '
    
        # sh -c code
        # THIS IS THE INTERESTING PART
        declare -A a;
        
        for ((i = 0; i < 3000; i++)); do
            a["$i"]=.; echo "$i";
        done
        # END INTERESTING PART
    
    ' | ts -s '%.s'
    done | awk '
    
        # we have: $timestamp $i
        # we want: $delta $i
        
        BEGIN { p_ts = 0 }
        
        { 
            if(p_ts > $1) {
                p_ts = 0
            }

            if(! N[$2]) {
                S[l++] = $2
            }

            X[$2] += ($1 - p_ts)
            N[$2]++

            p_ts = $1
        }

        END {
            for(i = 0; i < l; i++) {
                k = S[i]
                printf "%f\t%s\n", X[k]/N[k], k
            }
        }
    ' > "sh-$i.dat"
done 

gnuplot /dev/stdin <<'PLOT' > "$tmpimg"
reset
set terminal png notransparent size 1200,960 

set xtics 512
set xlabel "item"

set ylabel "time (ms)"

set title "hash_insert performance"

set style data linespoints

plot "sh-0.dat" using 2:($1*1000) title "master", \
     "sh-1.dat" using 2:($1*1000) title "4.3", \
     "sh-2.dat" using 2:($1*1000) title "devel", \
     "sh-3.dat" using 2:($1*1000) title "devel no-bash-malloc"
PLOT

echo "image: $tmpimg"




I compiled the shells like this:

/tmp/bash/master-O2/bin/bash:
    CFLAGS='-O2' ./configure --prefix=/tmp/bash/master-O2 && make install
/tmp/bash/4.3-rc2-O2/bin/bash:
    CFLAGS='-O2' ./configure --prefix=/tmp/bash/4.3-rc2-O2 && make install
/tmp/bash/devel-O2/bin/bash:
    CFLAGS='-O2' ./configure --prefix=/tmp/bash/devel-O2 && make install
/tmp/bash/devel-nobashmalloc-O2/bin/bash:
    CFLAGS='-O2' ./configure --with-bash-malloc=no 
--prefix=/tmp/bash/devel-nobashmalloc-O2 && make install


I attach the graph that results from running the script. And for those who
check this through the archive, I leave a link to it here:

    http://i.imgur.com/ir8zJ6s.png

As you can notice, there's a weird spike for both the 4.3 and devel versions
which use the internal bash malloc around 2560.

Attachment: graph.png
Description: PNG image


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