Performance does not come without a price. And the optimization process is more like a journey finding the balance between cost, security and performance.

After the system is up and running, there is something need to tweak according the workload to achieve better performance.

You could use sysctl -w key=value or write to the proc fs, after that, validate the system behaves as you expected, if yes, then you may write the configuration to /etc/sysctl.conf

Table of Contents


The USE Method

  • Utilization
  • Saturation
  • Errors
[] [] [] [] []   ---------------
-------------->  |             |
                 | Utilization |
   o o x o o     |             |
<-------------   ---------------

Resource Types

  • I/O Resources
  • Capacity Resources

Software Resources

  • Mutex Locks
  • Thread Pools
  • Process/Thread Capacity
  • File Descriptor Capacity

Monitoring & Benchmarking

Data Collection

classic time window:

  • high peak under pressure
  • random duration under normal pressure

The proc File System

System Monitor

Command-line Tools

  • top
  • vmstat
  • uptime/w
  • ps, pstree
  • free
    • total: physical memory - (s small bit that the kernel permanatly reserved for itself at startup)
    • used: memory in use by the OS
    • free: memory not in use
    • total = used (shared, buffers, cached, …) + free
    • -/+ buffers/cache: used - (buffers + cached)/free + (buffers + cached)
    • cached
      • result of completed I/O operations
      • tmpfs
  • iostat (I/O bound)
    • system I/O device loading
  • sar
    • CPU
      • sar -u -P ALL -f /var/log/sa/sa24
      • asr -q -P ALL -f …
    • swap: sar -S
    • memory: sar -{R,r}
    • task queue: sar -q
    • network: sar -n DEV
  • mpstat (CPU bound)
  • numastat
    • memory statistics for processes and the operating system on a per-NUMA node basis
  • numad
    • an automatic NUMA affinity management daemon
  • pmap
  • netstat
    • netstat -i
    • netstat -s
  • ss
    • statistics information about sockets
    • ss -s
  • ip
    • ip -s link
  • tcpdump/ethtool
  • strace
  • optional install
    • iptraf
    • nmon
    • iotop
    • dstat
  • turbostat
    • Intel Trubo Boost Tech
    • processor topology, frequency
    • idle power-stat, statistics
    • temperature, power usage
  • irqbalance
    • distributes hardware interrupts accross processors to improve system performance

Application Profilers


Tuning and probing, more deeper, more precise




A system-wide performance monitoring tool


Detection and profiling tools to help improve performance of application


  • A profiler tool for Linux 2.6+
  • Based on the perf events interfaces exported by Linux kernel

Perf data sources:

  • hardware performance counters
    • enable performance counter for virtual machine
  • kernel tracepoints

Becnmark Tools

netperf/iperf/iometer/ttcp/ab/Apache Jmeter/bonnie

Load Generator

Monitor Performance

Monitor System Utilization


Analyzing System Performance


  1. known the system (gather system information)
  2. backup
  3. monitor and analying the system’s performance
  4. narrow down the bottleneck and find its cause
  5. fix the bottleneck cause by trying one change at a time
  6. go back to step 3 until satisfied with the performance

Incase of

  • positive false
  • negative false

Generic Tuning

a number of different profiles optimized for different workloads to maximize the performance respectively
[root@rhel.vmg will]# tuned-adm list
Available profiles:
- balanced
- desktop
- latency-performance
- network-latency
- network-throughput
- powersave
- throughput-performance
- virtual-guest
- virtual-host
Current active profile: network-latency


schematic interaction of different performance componenets

|                  Applications                     |
| Libraries |                                       |
|                                                   |
|  Kernel                                           |
|                      -----------------------------|
|                      |          Drivers           |
|            ---------------                        |
|            |   Firmware  |                        |
|                     Hardware                      |



Configure kernel tick time

setting hardware performance policy


process life cycle

   ------------------              wait()               ------------------
-> | parent process |- - - - - - - - - - - - - - - - - >| parent process | ->
   ------------------                                   ------------------
          |                                                   ^
          | fork()                                            |
          v                                                   |
   ------------------  exec()  -----------------  exit() ------------------
   |  child process |--------->| child process |-------->| zombie process |
   ------------------          -----------------         ------------------

process priority

  • static
    • -20 ~ 19
    • nice, renice
    • requires root privilege to increse
  • dynamic

context switch

  • the context of the running process is stored
  • and the context of the next running process is restored to the registers

The process descriptor and the area are called kernel mode stack.

Process State

running or waiting to run in the queue (run queue)
suspended by certain signals (SIGINT, SIGSTOP), waiting to be resumed by a signal such as SIGCONT
suspended and wait for a certain condition to be satisfied (example, waiting for keyboard input)
sending a signal does nothing to the process in this state (example, waiting for disk I/O)
the process is waiting for its parent to be notified to release all the data structure. processes in this state could not be killed, could kill its parent instead
                                                  | TASK_ZOMBIE |
    fork()                                            exit()
       |                                                ^
       v                                                |
----------------           scheduling           ------------------
| TASK_RUNNING |  ----------------------------> | TASK_RUNNING   |
|    (READY)   |  <---------------------------- | (on processor) |
----------------           preemption           ------------------
       ^                                                v
       |                                                |
       |<----------<   TASK_STOPPED          <----------|
       |<----------<   TASK_INTERRUPTIBLE    <----------|
       |<----------<   TASK_UNINTERRUPTIBLE  <----------|

Schedule Policies

  • Realtime policies

    defines a fixed priority (1 ~ 99) for each thread

      • referred to as static priority scheduling
    • SCHED_RR
      • a round-robin variant of the SCHED_FIFO
      • threads with the same priority are scheduled round-robin style within a certain quantum, or time slice: sched_rr_get_interval(2)
      • but the duration of the time slice cannot be set by a user
      • this policy is useful if you need multiple thread to run at the same priority
  • Normal policies

    Both SCHED_BATCH and SCHED_IDLE are intended for very low priority jobs, and as such are of limited interest in a performance tuning topic.

      • use Completely Fair Scheduler (CFS) to provide fair access periods for all threads using this policy
      • CFS establishes a dynamic priority list partly based on the niceness value of each process thread
      • this gives users some indirect level of control over process priority
      • but the dynamic priority list can only be directly changed by the CFS


  • setting process affinity with taskset
  • managing NUMA affinity with numactl
  • automatic NUMA affinity management with numad

Isolate CPUs: isolacpus boot parameter, prevent any user space threads on these CPUs

Tuna can isolate a CPU at any time

Interrupts and IRQ


  • soft
  • hard

Binding interrupts to a single physical processor could improve system performance.

setting interrupts affinity: /proc/irq/irq_number/smp_affinity


Configuring CPU, thread, and interrupt affinity with Tuna

Performance Metrics

  • CPU Utilization
  • User Time
    • Depicts the CPU percentage spent on user process, including nice time
  • System Time
    • IRQ and softing time
    • High and sustained system time values can point bottlenecks in the network and driver stack
    • A system should spent as little time as possible in kernel time
  • Waiting
    • Total amount of time spent waiting for an I/O operation to occur
    • A system should not spend too much time waiting for I/O operation
  • Idel Time
  • Nice Time
    • Depicts the CPU percentage spent on re-nicing processes that change the execution order and priority of processes
  • Load Average
    • The load average is not a percentage, but the rolling average of the sum of the following:
      • the number of processes in queue waiting to be processed
      • the number of processes waiting for uninterruptible task to complete
    • This is the average of the sum of TASK_RUNNING and TASK_UNINTERRUPTIBLE processes
  • Runnable Processes
    • processes that are ready to be executed
    • should not exceeds 10 times of the amount of physical processors for a sustained peroid of time
  • Blocked
    • waiting for I/O operation to finish
  • Context Switches
    • Amount of switches between threads that occur on the system context switches generally are not desirable because the CPU cache is flushed with each one, but some are necessary
  • Interrupts
    • Contains hard and soft ones
    • Hard interrupts have a more adverse effect on system performance
    • Interrupts value includes the interrupts caused by the CPU clock


  • Tuning process priority
    • nice
    • renice
  • CPU affinity for interrupt handling
    • bind processes that cause a significant amount of interrupts to a CPU
    • Let physical processors handle interrupts
  • Considerations for NUMA systems
    • numastat
    • /sys/devices/system/node/{nodenum}/numastat
    • NUMA affinity


Memory Management

|                           physical memory                                 |
|            page-level allocator                |      space for kernel    |
|------------------------------------------------|                          |
|  KMA(kernel memory allocator) | paging system  | * codes                  |
|-------------------------------|----------------|                          |
| * net buffer                  | * user process | * static data structures |
| * procfs                      | * block cache  |                          |
| * inodb,file handle           |                |                          |

Process Memory Segments

process address space

|               Text                 |
| Executable instructions (Read Only)|
|               Data                 |    |
|         Initialized Data           |    |
|------------------------------------|    |
|                BSS                 |     > Data Segment
|       Zero-Initialized Data        |    |
|------------------------------------|    |
|               Heap                 |    |
|           Dynamic Memory           |    |
|        Allocated by malloc()       |    |
|                                    |
|              Stack                 |
|       * Local Variables            |
|       * Function Parameters        |
|       * Return Address, etc        |
report memory map of a process

Performance Metrics

  • Free memory
    • substract the amount of buffers and cache from the used memory to determine (effectively) free memory
  • Swap usage
    • swap in/out is a reliable means of identifying a memory bottleneck
  • Buffer and cache
    • cache allocated as file system and block device cache
  • Slabs
    • depicts the kernel usage of memory
    • note that the kernel page could not be paged out to disk
  • Active vs inactive memory
    • provides information about the active use of the system memory
    • inactive memory is a likely candidate to be swapped out to disk by the kswapd daemon


Page Size

  • default 4KB
  • static huge page <= 1GB
  • transparent huge page 2MB

Transaltion Lookaside Buffer (TLB) size

allow memory to be managed in very large segments

Monitoring and Diagnosing Performance Probes

monitoring memroy usage: vmstat

Huge Pages


Profiling application memory usage with Valgrind


Memcheck (Memory Usage)

Cachegrind (Cache Usage)

Massif (Heap and Stack Space)


  • huge pages /proc/sys/vm/nr_hugepages
  • system memory capacity
    • dirty_ratio
    • dirty_background_ratio
    • overcommit_memory
    • overcommit_ratio
    • max_map_count


  • min_free_kbytes
  • oom_adj
  • swappiness


Virtual Memory

# Do less swapping
vm.swappiness = 10
vm.dirty_ratio = 60
vm.dirty_background_ratio = 2



setting kernel swap and pdflush behavior

  • /proc/sys/vm/swappiness
    • can be used to define how aggressively memory pages are swapped to disk
  • /proc/sys/vm/dirty_background_ratio
    • defines at what percentage of main memory the pdflush daemon should write data out to the disk
  • /proc/sys/vm/dirty_ratio
    • defines at what level the actual disk writes will take place
    • this value is a percentage of main memory

Swap partition

Linux also uses swap space to page memory areas to disk that have not been used for a significant amount of time


This memory management feature is valuable for applications that use a large virtual memory address space. It is especially useful for database applications.

The CPU’s Translation Lookaside Buffer (TLB) is a small cache used for storing virtual-to-physical mapping information.

For simplicity, this feature is exposed to applications by means of a file system interface.

Disk I/O


I/O subsystem architecture

I/O stack

Block I/O on Linux

                      File I/O          File I/O
User Space                ^                 ^
Kernel Space              |                 |
                  | Virtual File System (VFS) Layer |
       | Individual File Systems (ext3, ext4, XFS, VFAT, ...) |
               |       Buffer Cache (Page Cache)      |
               |             I/O Schedulers           |
               |    cfq   /   deadline   /    noop    |
        Request Queue |                      | Request Queue
               -------v--------        ------v---------
               | Block Driver |        | Block Driver |
               -------^--------        ------^---------
Kernel Space          |                      |
Storage Media         |                      |
                      v                      v
                  ----------            ------------
                  |  Disk  |            | CD-Drive |
                  ----------            ------------


  • Locality of reference
    • temporal locality
      • the data most recently used has a high probablity of being used in the near future
    • spartial locality
      • the data that resides close to the data which has been used has a high probablity of being used

Linux use this principle in many conponments such as page cache, file object cache (i-node cache, directory entry cache, etc), read ahead buffer and more.


pdflush runs

  • on a regular basic (kupdate)
  • when the proportion of dirty buffers exceeds a certain threhold (bdflush), the threhold is configurable in the /proc/sys/vm/dirty_background_ratio (5 by default)

Block Layer

The block layer handles all the activity related to the block device operation

bio is the key data structure, an interface between the file system layer and the block layer)

Block size
the smallest amount of data that can be read or written to a drive, can have a direct impact on a server’s performance. Reformat is needed to change the block size.


The I/O scheduler are now selectable on a per-disk basis.

  • noop
    • No Operation, simple and lean
    • a simple FIFO queue that does not perform any data ordering
    • simply merges adjacent data requests
    • assumes that a block device either
      • features its own elevator algorithm such as TCQ for SCSI
      • or that the block device has no seek latency such as a flash card
    • often the best choice for memory-backed block devices (e.g. ramdisks) and other non-rotational media (flash) where trying to reschedule I/O is a waste of resources
  • deadline
    • a cycle elevator (round robin) with a deadline algorithm that provides a near real-time behavior of the I/O system
    • a lightweight scheduler which tries to put a hard limit on latency
    • offers excellent request latency while maintaining good disk throughput
    • ensures that starvation of a process cannot happen
    • better for solid state disks (SSD)
  • cfq (Completely Fair Queuing)
    • implements a QoS policy for processes by maintain per-process I/O queues
    • tries to maintain system-wide fairness of I/O bandwidth
    • aggresively attempts to avoid starvation of processes and features low latency
    • well suited for large multi-user systems with a lot of competing processes
    • better for physical spinning storage devices
    • can slowdown a single main application (e.g. database)

To set a specific scheduler, simply do this:

echo SCHEDNAME > /sys/block/DEV/queue/scheduler

where SCHEDNAME is the name of a defined IO scheduler, and DEV is the device name (hda, hdb, sga, or whatever you happen to have).

The list of defined schedulers can be found by simply doing a cat /sys/block/DEV/queue/scheduler - the list of valid names will be displayed, with the currently selected scheduler in brackets:

# cat /sys/block/hda/queue/scheduler
noop deadline [cfq]
# echo deadline > /sys/block/hda/queue/scheduler
# cat /sys/block/hda/queue/scheduler
noop [deadline] cfq

I/O Device Driver

The Linux kernel takes control of devices using a device driver. The device driver is usually a separate kernel module and and is privided for each device (or group of devices) to make the device available for the Linux OS.

Performance Metrics

  • iowait
    • time the CPU spends waiting for an I/O operation to occur
  • average queue length
    • amount of outstanding I/O requests
    • in general, a disk queue of 2~3 is optimal
  • average wait
    • a measurement of the average time in ms it takes for an I/O request to be serviced
    • the wait time consists of the actual I/O operation and the time it waited in the I/O queue
  • transfer per second (read and write per second)
    • the transfer per second metric in conjunction with the KBytes/s value helps to identify the average transfer size of the system
    • the average transfer size should match with the strip size used by the disk system
  • blocks read/write per second
    • expressed in blocks of 1024 bytes as of kernel 2.6
  • kilobytes per second read/write


Enable asynchronous I/O and Direct I/O support

provides a native Linux asynchronous I/O api

Tuning Async I/O:

  • aio-nr
    • shows the current system-wide number of asynchronous io requests
  • aio-max-nr
    • allows change the maximum value aio-nr can grow to
  • /proc/sys/fs/epoll
    • max_user_watches

File System


  • Btrfs
  • Global File System 2
  • Network File System
  • FS-cache

File Hierarchy Standard (FHS)

Virtual File System

is an abstraction interface layer that resides between the user process and various types of Linux file system implementations.

VFS provides common object models (such as i-node, file object, page cache, directory entry, etc) and methods to access file system objects.

VFS concepts

User Process -< cp
System Call -< open(), read(), write()
    VFS     -< translation foreach file system
 ext2 | NFS | ext3 | VFAT | XFS | proc

Journaling Concepts


  1. write journal logs
  2. make changes to actual file system
  3. delete journal logs
| Journal Area |    File System   |

Formatting Options

Mount Options


File System Formats



  • extend-based file system
    • if possible, files extent allocation map stored in its inode
  • stripe geometry
    • su : strip unit (chunk size)
    • sw : strip width (number of strip in the strip)
  • default atime behavior is relatime
  • size option
    • reduce: not supported
    • enlarge: xfs_growfs
  • write barriers
    • ensure file system integerity
    • nobarrier applies if
      • without write cache
      • battery-backend
  • delayed allocation
    • reduce fragmentation
    • increse performance
  • XFS support extended attributes for files
  • direct I/O -> DMA
    • high throughput
    • non-cached I/O
  • external XFS Journals
    • SSD
    • -logdev=device,size=size
allocation groups
virtual storage regions of fixed size
allocation group
* own set of inodes
* free space
* scalability
* parallelism I/O
fs.file-max = 2097152

Above setting is specified for system-wide configuration. For user, there is a configure item in /etc/security/limits.conf named nofile, which means max number of open file descriptors.

prlimit and ulimit can be used to inspect the limit: ulimit -<H|S>n or prlimit -n


See performance-tuning-networking for more detail

Resource Limits

ulimit provides control over resources available to each user via a shell. You can type ulimit -a to get a list of all current settings. In parentheses you will see one or two items: the units in measurements (e.g. kbytes, blocks, seconds) as well as a letter option (e.g. -s, -t, -u). The letter option will let you view/edit one particular setting at a time.

  • ulimit -S -a view all soft limits
  • ulimit -H -a view all hard limits
  • ulimit -S [option] [number] set a specific soft limit for one variable

    e.g. ulimit -S -s 8192 set a new soft stacksize limit, “-s” is for stack

  • ulimit -H [option] [number] set a specific hard limit for one variable

    e.g. ulimit -H -s 8192 set a new hardstacksize limit, “-s” is for stack

  • /etc/security/limits.conf file where you can set soft and hard limits per user or for everyone

    e.g. Add in the following to /etc/security/limits.conf will set a soft stacksize of 8192 adn a hard stacksize of unlimited for username “toor”:

    toor soft stack 8192
    toor hard stack unlimited
[root@dns will]# ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 7823
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 7823
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

limit type


for enforcing hard resource limits. These limits are set by the superuser and enforced by the Kernel. The user cannot raise his requirement of system resources above such values.


for enforcing soft resource limits. These limits are ones that the user can move up or down within the permitted range by any pre-existing hard limits. The values specified with this token can be thought of as default values, for normal system usage. from limits.conf(5)


man pages

  • getrlimit(2)
  • setrlimit(2)

Web Resources