This week's book giveaway is in the Python forum.
We're giving away four copies of High Performance Python for Data Analytics and have Tiago Rodrigues Antao on-line!
See this thread for details.
Win a copy of High Performance Python for Data Analytics this week in the Python forum!
  • Post Reply Bookmark Topic Watch Topic
  • New Topic
programming forums Java Mobile Certification Databases Caching Books Engineering Micro Controllers OS Languages Paradigms IDEs Build Tools Frameworks Application Servers Open Source This Site Careers Other all forums
this forum made possible by our volunteer staff, including ...
Marshals:
  • Campbell Ritchie
  • Paul Clapham
  • Ron McLeod
  • Bear Bibeault
  • Liutauras Vilda
Sheriffs:
  • Jeanne Boyarsky
  • Tim Cooke
  • Junilu Lacar
Saloon Keepers:
  • Tim Moores
  • Tim Holloway
  • Stephan van Hulst
  • Jj Roberts
  • Carey Brown
Bartenders:
  • salvin francis
  • Frits Walraven
  • Piet Souris

I/O waiting CPU time – ‘wa’ in top

 
Ranch Hand
Posts: 63
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
CPU consumption in Unix/Linux operating systems are studied using 8 different metrics: User CPU time, System CPU time, nice CPU time, Idle CPU time, Waiting CPU time, Hardware Interrupt CPU time, Software Interrupt CPU time, Stolen CPU time. In this article let us study about ‘waiting CPU time’.

What is ‘waiting’ CPU time?
Waiting CPU time indicates the amount of time CPU is waiting for the disk I/O or network I/O operations to complete. High waiting time indicates that the CPU is *stranded* because of the I/O operations on that device. For optimal performance, one should aim to keep the I/O waiting CPU time as low as possible. If waiting time is > 10% then it is worth investigating it.

You can visualize I/O waiting time through this analogy: Say there are hundreds of cars/bikes are waiting on a busy road for the traffic light to switch from ‘red’ to ‘green’. But due to some technical glitch, it takes  long time for the traffic light to switch from ‘red’ to ‘green’ – then those hundreds of cars/bikes would get stranded unnecessarily. It will result in several undesirable side effects: passengers will reach their destination late, drivers can get frustrated and start to horn (noise pollution),  since engines are on fuel will be wasted (air pollution)…

How to find ‘waiting’ CPU time?
Waiting CPU time can be found from the following sources:

a. You can use web-based root cause analysis tools like yCrash to report ‘waiting’ CPU time. Tool is capable of generating alerts if ‘waiting’ CPU time goes beyond the threshold.

b. ‘waiting’ CPU time is also reported in the Unix/Linux command line tool ‘top’ in the field ‘wa’ as highlighted in the below image.


              Fig: ‘wa’ time in top

How to simulate high ‘waiting’ CPU time?
To simulate high ‘waiting’ CPU reporting let’s use BuggyApp. BuggyApp is an opensource java project which can simulate various sort of performance problems. When you launch  BuggyApp with following arguments, it will cause the ‘waiting’ CPU consumption to spike up on the host.





            Fig: You can see the waiting CPU time spike up to 75.9%

Now let’s see the source code in the BuggyApp which is causing the ‘waiting’ CPU time to spike up.



Here you can see that BuggyApp is launching 5 ‘IOThreads’ in ‘IODemo’ class. You can notice that ‘IOThread’ is going on an infinite while loop. In that loop it’s writing the content in to a file and reading the same content from the file. It is doing these 2 steps repeatedly again and again. Since writing contents and reading contents from the disk is a heavy I/O intensive operation ‘waiting’ CPU time is spiking up to 75.9%

How to resolve high ‘waiting’ time?
In case if your device is suffering from high I/O waiting time then you can follow the steps outlined below to reduce the I/O waiting time:

1. You can use the root cause analysis tools like yCrash which will point the lines of code in the application which is causing the high I/O waiting time.
2. You can optimize the application’s waiting time by doing the following:

* Reduce the number of database calls
* Optimize the database queries such that less data is returned from DB to app
* Reduce the number of network calls that is made to external applications
* Try to minimize the amount of payload that is sent between external applications and your application
* Try to reduce number of files that written to disk.
* Try to reduce amount of data read from the disk.
* Make sure only the essential log statements are written into the disk.

3. Make sure your OS is running on the latest version with all patches installed. This is not only good from the security perspective, but it will also improve the performance.
4. Make sure sufficient free memory is allocated on the device. Lack of free memory has two detrimental effects:

* If there is a lack of free memory, then processes will be swapped in and out of memory. Several pages will be written and read from the disks frequently. It will increase the disk I/O operations.
* If there is less free memory, then OS wouldn’t be able to cache frequently used disk blocks in memory. When frequently used disk blocks are cached, I/O waiting time will go down.

5. Keep your filesystem disk usage below 80% to avoid excessive fragmentation. When there is excessive disk fragmentation, I/O time will increase.
6. If all of the above steps fail, you may also consider upgrading your storage for better performance. You might consider switching to faster SSD, NVMe, SAN storage,…


 
Saloon Keeper
Posts: 12628
273
  • Mark post as helpful
  • send pies
    Number of slices to send:
    Optional 'thank-you' note:
  • Quote
  • Report post to moderator
I have moved your post to Blatant Advertising. Please post there directly next time.
 
Be reasonable. You can't destroy everything. Where would you sit? How would you read a tiny ad?
Building a Better World in your Backyard by Paul Wheaton and Shawn Klassen-Koop
https://coderanch.com/wiki/718759/books/Building-World-Backyard-Paul-Wheaton
reply
    Bookmark Topic Watch Topic
  • New Topic