Ubuntu Manpage: fileslower - Trace slow synchronous file reads and writes.

Provided by: bpfcc-tools_0.18.0+ds-2_all

NAME

       fileslower - Trace slow synchronous file reads and writes.

SYNOPSIS

       fileslower [-h] [-p PID] [-a] [min_ms]

DESCRIPTION

       This  script  uses  kernel  dynamic  tracing  of  synchronous  reads and writes at the VFS
       interface, to identify slow file reads and writes for any file system.

       This version traces __vfs_read() and __vfs_write() and only showing synchronous  I/O  (the
       path  to  new_sync_read()  and  new_sync_write()),  and  I/O with filenames. This approach
       provides a view of just two file system request types: file reads and  writes.  There  are
       typically  many  others:  asynchronous  I/O, directory operations, file handle operations,
       file open()s, fflush(), etc.

       WARNING: See the OVERHEAD section.

       By default, a minimum millisecond threshold of 10 is used.

       Since this works by tracing various kernel __vfs_*() functions using dynamic  tracing,  it
       will need updating to match any changes to these functions. A future version should switch
       to using FS tracepoints instead.

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

REQUIREMENTS

       CONFIG_BPF and bcc.

OPTIONS

       -p PID Trace this PID only.

       -a     Include non-regular file types in output (sockets, FIFOs, etc).

       min_ms Minimum I/O latency (duration) to trace, in milliseconds. Default is 10 ms.

EXAMPLES

       Trace synchronous file reads and writes slower than 10 ms:
              # fileslower

       Trace slower than 1 ms:
              # fileslower 1

       Trace slower than 1 ms, for PID 181 only:
              # fileslower -p 181 1

FIELDS

       TIME(s)
              Time of I/O completion since the first I/O seen, in seconds.

       COMM   Process name.

       PID    Process ID.

       D      Direction of I/O. R == read, W == write.

       BYTES  Size of I/O, in bytes.

       LAT(ms)
              Latency (duration) of I/O, measured from when the application issued it to  VFS  to
              when  it  completed.  This  time  is inclusive of block device I/O, file system CPU
              cycles, file system locks, run queue latency, etc. It's a more accurate measure  of
              the  latency  suffered  by applications performing file system I/O, than to measure
              this down at the block device interface.

       FILENAME
              A cached kernel file name (comes from dentry->d_name.name).

OVERHEAD

Depending on the frequency of application reads and writes, overhead can become severe, in
the worst case slowing applications by 2x. In the best case, the overhead is negligible.
Hopefully for real world workloads the overhead is often at the lower end of the spectrum
-- test before use. The reason for high overhead is that this traces VFS reads and writes,
which includes FS cache reads and writes, and can exceed one million events per second if
the application is I/O heavy. While the instrumentation is extremely lightweight, and uses
in-kernel eBPF maps for efficient timing and filtering, multiply that cost by one million
events per second and that cost becomes a million times worse. You can get an idea of the
possible cost by just counting the instrumented events using the bcc funccount tool, eg:

# ./funccount.py -i 1 -r '^__vfs_(read|write)$'

This also costs overhead, but is somewhat less than fileslower.

If the overhead is prohibitive for your workload, I'd recommend moving down-stack a little
from VFS into the file system functions (ext4, xfs, etc). Look for updates to bcc for
specific file system tools that do this. The advantage of a per-file system approach is
that we can trace post-cache, greatly reducing events and overhead. The disadvantage is
needing custom tracing approaches for each different file system (whereas VFS is generic).

SOURCE

       This is from bcc.

              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

       Brendan Gregg