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pcstat - get page cache statistics for files

A common question when tuning databases and other IO-intensive applications is, "is Linux caching my data or not?" pcstat gets that information for you using the mincore(2) syscall.

The fincore application from linux-ftools does the same thing and I read its source code. I chose not to use it because it appears to be abandoned and has some build problems.

I wrote this is so that Apache Cassandra users can see if ssTables are being cached. If $GOPATH/bin is in your PATH, this will get it installed:

go install github.com/tobert/pcstat@latest
pcstat /var/lib/cassandra/data/*/*/*-Data.db

If you don't want to mess around with building the software, binaries are provided in orphaned branches so you can pull them down from Github with curl/wget.

if [ $(uname -m) == "x86_64" ] ; then
    curl -L -o pcstat https://github.com/tobert/pcstat/raw/2014-05-02-01/pcstat.x86_64
else
    curl -L -o pcstat https://github.com/tobert/pcstat/raw/2014-05-02-01/pcstat.x86_32
fi
chmod 755 pcstat
./pcstat /var/lib/cassandra/data/*/*/*-Data.db

Usage

Command-line arguments are described below. Every argument following the program flags is considered a file for inspection.

pcstat <-json <-pps>|-terse|-default> <-nohdr> <-bname> file file file
 -json output will be JSON
   -pps include the per-page information in the output (can be huge!)
 -terse print terse machine-parseable output
 -histo print a histogram using unicode block characters
 -nohdr don't print the column header in terse or default format
 -bname use basename(file) in the output (use for long paths)
 -plain return data with no box characters
 -unicode return data with unicode box characters

Examples

Default output

The default output is designed to be easy for humans to read at a glance and should look nice in any fixed-width font.

atobey@brak ~ $ pcstat testfile3
|-----------+----------------+------------+-----------+---------|
| Name      | Size           | Pages      | Cached    | Percent |
|-----------+----------------+------------+-----------+---------|
| LICENSE   | 11323          | 3          | 0         | 000.000 |
| README.md | 6768           | 2          | 2         | 100.000 |
| pcstat    | 3065456        | 749        | 749       | 100.000 |
| pcstat.go | 9687           | 3          | 3         | 100.000 |
| testfile3 | 102401024      | 25001      | 60        | 000.240 |
|-----------+----------------+------------+-----------+---------|

Terse output

Meant to be machine readable and easy to process with standard shell tools and scripts. Note: No attempt is made to escape characters for proper CSV at this time.

pcstat -terse -bname *
name,size,timestamp,mtime,pages,cached,percent
LICENSE,11323,1400767725,1400492571,3,0,0
README.md,6185,1400767725,1400767719,2,2,100
pcstat,3065456,1400767725,1400766869,749,749,100
pcstat.go,9687,1400767725,1400766807,3,3,100
testfile3,102401024,1400767725,1400761247,25001,60,0.23999040038398464

JSON output

The 'status' field will always be empty unless you add the -pps flag, which will cause status to be populated with an array of booleans, one per page in the file indicated whether it's cached or not. This can get spammy with big files so it's off by default.

atobey@brak ~ $ pcstat -json testfile3 |json_pp
[
 {
   "filename":  "testfile3",
   "size":      102401024,
   "timestamp": "2014-05-22T13:57:19.971348936Z",
   "mtime":     "2014-05-22T12:20:47.940163295Z",
   "pages":     25001,
   "cached":    60,
   "uncached":  24941,
   "percent":   0.23999040038398464,
   "status":    []
 }
]

Histogram output

Your terminal and font need to support the Block Elements section of Unicode for this to work. Even then, the output is inconsistent in my testing, so YMMV. See http://www.unicode.org/charts/PDF/U2580.pdf

The number after the filename is the number of pages in the file. This might be removed in the future.

atobey@brak ~ $ pcstat -bname -histo *
LICENSE          3 ▁▁▁
README.md        2 ██
pcstat         749 █████████████████████████████████████████████████
pcstat.go        3 ███
testfile      2560 ▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁
testfile2        3 ▁▁▁
testfile3    25001 ▂▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁▁

Testing

The easiest way to tell if this tool is working is to drop caches and do reads on files to get things into cache.

atobey@brak ~/src/pcstat $ dd if=/dev/urandom of=testfile bs=1M count=10
10+0 records in
10+0 records out
10485760 bytes (10 MB) copied, 0.805698 s, 13.0 MB/s
atobey@brak ~/src/pcstat $ ./pcstat testfile
|--------------------+----------------+------------+-----------+---------|
| Name               | Size           | Pages      | Cached    | Percent |
|--------------------+----------------+------------+-----------+---------|
| testfile           | 10485760       | 2560       | 2560      | 100     |
|--------------------+----------------+------------+-----------+---------|
atobey@brak ~/src/pcstat $ echo 1 |sudo tee /proc/sys/vm/drop_caches
1
atobey@brak ~/src/pcstat $ ./pcstat testfile
|--------------------+----------------+------------+-----------+---------|
| Name               | Size           | Pages      | Cached    | Percent |
|--------------------+----------------+------------+-----------+---------|
| testfile           | 10485760       | 2560       | 0         | 0       |
|--------------------+----------------+------------+-----------+---------|
atobey@brak ~/src/pcstat $ dd if=/dev/urandom of=testfile bs=4096 seek=10 count=1 conv=notrunc
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.000468208 s, 8.7 MB/s
atobey@brak ~/src/pcstat $ ./pcstat testfile
|--------------------+----------------+------------+-----------+---------|
| Name               | Size           | Pages      | Cached    | Percent |
|--------------------+----------------+------------+-----------+---------|
| testfile           | 10485760       | 2560       | 1         | 0       |
|--------------------+----------------+------------+-----------+---------|

Building

git clone https://github.com/tobert/pcstat.git
cd pcstat
go build
sudo cp -a pcstat /usr/local/bin
pcstat /usr/local/bin/pcstat

Requirements

Go 1.17 or higher.

From the mincore(2) man page:

  • Available since Linux 2.3.99pre1 and glibc 2.2.
  • mincore() is not specified in POSIX.1-2001, and it is not available on all UNIX implementations.
  • Before kernel 2.6.21, mincore() did not return correct information some mappings.

Author

A. Tobey [email protected] @[email protected]

License

Apache 2.0