Manual: fileslower-bpfcc

fileslower(8) System Manager's Manual fileslower(8)

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 ap‐
plications 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 advan‐
tage of a per-file system approach is that we can trace post-cache, greatly reducing events and overhead. The disad‐
vantage 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

Manual: fileslower-bpfcc

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