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)