NAME

       readahead - Show performance of read-ahead cache

SYNOPSIS

       readahead [-d DURATION]

DESCRIPTION

       The  tool  shows the performance of read-ahead caching on the system under a given load to
       investigate any caching issues. It shows a count of unused pages in  the  cache  and  also
       prints a histogram showing how long they have remained there.

       This  tool  traces the __do_page_cache_readahead() kernel function to track entry and exit
       in  the  readahead  mechanism  in  the  kernel  and  then  uses  __page_cache_alloc()  and
       mark_page_accessed()  functions  to  calculate the age of the page in the cache as well as
       see how many are left unaccessed.

       Since this uses BPF, only the root user can use this tool.

   NOTE ON KPROBES USAGE
       Since the tool uses Kprobes, depending on your linux kernel's compilation, these functions
       may  be inlined and hence not available for Kprobes. To see whether you have the functions
       available, check vmlinux source and binary to confirm whether  inlining  is  happening  or
       not.  You can also check /proc/kallsyms on the host and verify if the target functions are
       present there before using this.

REQUIREMENTS

       CONFIG_BPF, bcc

OPTIONS

       -h Print usage message

       -d DURATION
              Trace the read-ahead caching system for DURATION seconds

EXAMPLES

       Trace for 30 seconds and show  histogram of page age (ms) in read-ahead cache  along  with
       unused page count:
              # readahead -d 30

OVERHEAD

       The kernel functions instrumented by this program could be high-frequency depending on the
       profile of the application (for example sequential IO). We advise the users to measure and
       monitor the overhead before leaving this turned on in production environments.

SOURCE

       This  originated  as  a  bpftrace tool from the book "BPF Performance Tools", published by
       Addison Wesley (2019):

              http://www.brendangregg.com/bpf-performance-tools-book.html

       See the book for more documentation on this tool.

       This version is in the BCC repository:

              https://github.com/iovisor/bcc

       Also look in the bcc distribution for a companion _examples.txt  file  containing  example
       usage, output, and commentary for this tool.

OS

       Linux

STABILITY

       Unstable - in development.

AUTHOR

       Suchakra Sharma

SEE ALSO

       readahead(2), madvise(2)