Bug zram Incorrect Calculation of CPUs & Swap Areas

SamK · posted: 2014-09-22 12:45

Noticed this months ago but did not get round to looking in to it until yesterday.

Hardware=Single CPU/core
Distro=antiX-13.2-Full-Stable
zram=copied to /etc/init.d/ from /usr/local/bin/
System=rebooted

Diagnosis
All following conducted with root privileges

Report the status of swapspace started by zram

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swapon -s
Filename                Type        Size    Used    Priority
/dev/zram0                              partition    95500    0    100
/dev/zram1                              partition    95500    0    100

This seems incorrect as two instances are reported.

The following line in /etc/init.d/zram calculates the number of CPUs

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...
num_cpus=$(grep -c processor /proc/cpuinfo)
...

It counts the number of times the word processor is present in the file /proc/cpuinfo

Verifying what the line returns in a terminal

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grep -c processor /proc/cpuinfo
2

This figure is used by the zram script to start the above two instances

Explanation of value returned

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grep --line-number processor /proc/cpuinfo
1:processor    : 0
5:model name    : Intel(R) Pentium(R) M processor 1.73GHz

The search term appears twice (lines 1 & 5) and thereby reports an invalid value of 2 CPUs on a single CPU/core system. It begins counting upwards from 1.

Later in the script a section is included to handle the incorrectly reported number of CPUs

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last_cpu=$((num_cpus - 1))

This decreases the incorrectly reported value of 2 to 1

The adjusted value is then fed into the start and stop sections

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for i in $(seq 0 $last_cpu); do...

This starts counting upwards from 0 (zero).

Outcome
Even though the number of CPUs has been adjusted (decreased by 1), an incorrect number of swap areas are created because the relevant section of the script starts counting from 0. A swap area is created for number 0, and a swap area is created for number 1.

Options
Workaround Fix

Retain incorrect calculation section
Retain section to compensate for incorrect calculation
Adjust start and stop sections to begin counting upwards from 1 to match incorrect sections

In start and stop sections of zram script change value from 0 to 1

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for i in $(seq 1 $last_cpu); do...

Full Fix

Correctly calculate number of CPUs
Jettison section to compensate for incorrect calculation
Adjust start and stop sections to begin counting upwards from 1 to match correct correct calculation section

Requires major rewrite.

Suggestions
antiX-13.2-Full-Stable ships with two suitable apps as components of packages which Debian marks as essential

getconf is part of libc-bin
nproc is part of coreutils

Verification that both return the correct CPU count in a terminal

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getconf _NPROCESSORS_ONLN
1
nproc
1
nproc --all
1

Verification of correct zram operation with any of the above commands substituted for the present command

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sudo swapon -s
Filename                Type        Size    Used    Priority
/dev/zram0                              partition    191004    0    100

Note: the size of the swap area is incorrect as the relevant part of the file was not edited for this quick and dirty test.

Of the two commands my personal preference is for one of the nproc variants because its format is familiar and adaptable. Additionally the online GNU manual indicates that in use"The result is guaranteed to be greater than zero". This will enable the following additional section to be jettisoned

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# if something goes wrong, assume we have 1
["$num_cpus" != 0 ] || num_cpus=1

.

References

========= SCRAPER REMOVED AN EMBEDDED LINK HERE ===========
url was:"https://www.gnu.org/software/coreutils/manual/html_node/nproc-invocation.html"
linktext was:"https://www.gnu.org/software/coreutils/ ... ation.html"
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========= SCRAPER REMOVED AN EMBEDDED LINK HERE ===========
url was:"http://www.unix.com/man-page/linux/1/getconf/"
linktext was:"http://www.unix.com/man-page/linux/1/getconf/"
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