dcsimg

Tuning CFQ – What Station is That?

The last article was a quick overview of the 4 schedulers in the Linux kernel. This article takes a closer look at the Completely Fair Queuing (CFQ) scheduler and how you can tune it.

The previous article was an overview of the four standard IO schedulers in the Linux kernel. These IO schedulers are very crucial in determining the IO performance of your hand-held, laptop, desktop, server (of various flavors), or supercomputer nodes. It’s possible to tune the IO scheduler for, possibly, better performance.

This article presents an example of one possible tuning option for the default IO scheduler, CFQ. It adjust just one parameter but the point is to show you that it’s possible and, more importantly, remind you to measure the performance both before and after the changes to determine the effect of the parameter change.

CFQ IO Scheduler

Recall that the CFQ scheduler was written in response to some potential problems with the deadline controller. In particular, from an interview with Jens Axboe, the developer of CFQ, deadline, and a few other IO schedulers, Jens stated, “While deadline worked great from a latency and hard drive perspective, it had no concept of individual process fairness.” So one process that was doing a great deal of IO could cause other applications to have their IO starve.

The CFQ scheduler created the concept of having queues for each process. These queues are created as needed for a particular process. Also, while perhaps not new, the scheduler divided the concept of IO into two parts, Synchronous IO and Asynchronous IO (AIO). Synchronous IO is important because the application stops running until the IO request is finished. In other words, synchronous IO “blocks” the execution of the application until it is done. This is fairly common in read operations because an application may need to read some input data prior to continuing execution.

On the other hand, AIO allows an application to continue operation because the result of the IO operation returns immediately. So rather than wait for confirmation that the application’s IO request has succeeded as in the case of synchronous IO, for AIO the result is returned immediately even if the IO operation has not yet finished. This is potentially very useful for write operations and allows the application to overlap IO with computation (helps efficiency – that is, it can improve run time).

CFQ was designed to separate out synchronous and asynchronous IO operations, favoring synchronous operations (naturally). It also favors read operations for a couple of reasons: (1) reads have a tendency to block execution because the application needs the data to continue, (2) it’s possible with the elevator approach for schedulers to “starve” a read operation that is fairly far out on disk geometry (near the outside of the disk). By favoring read operations it improves read responsiveness and greatly reduces the possibility of a far-out read starvation.

CFQ goes even further by keeping the concept of deadlines from the Deadline IO Scheduler to prevent IO operations from being starved. Jens’ wrote the deadline scheduler and realized that for good performance for some applications it needed the concept of an IO operation “timing out.” That is, an IO operation may be put into a queue for execution but subsequent IO operations may be put ahead of it in the queue. Therefore the IO request at the end of the queue may never get executed or it’s execution may be seriously delayed. The deadline IO scheduler has the concept of a “time-out” period. If the IO request is not executed in during this period, it will be executed immediately. This keeps IO operations from starving in the queue.

Jens combined all of these concepts along with the per-process concept to create CFQ. It can be rather complicated to define exactly how these concepts interact and it’s beyond this article to go into detail, but understanding the concepts that go into CFQ are very important. This is particularly important if we are going to try tuning the scheduler for performance.

Tunable Parameters in CFQ

In addition to open-source being a great way to have access to the code to make changes yourself to adapt it to your requirements, many times the developers of software allowing you to “tune” the application for your situation without having to hack the code base yourself. The IO schedulers in the Linux kernel are no exception. In particular, the CFQ scheduler has 9 parameters for tuning performance. While discussing the parameters can get long, it is worthwhile to take a look at these parameters in a bit more depth:


  1. back_seek_max
    This parameter, given in Kbytes, sets the maximum “distance” for backward seeking. By default, this parameter is set to 16 MBytes. This distance is the amount of space from the current head location to the sectors that are backward in terms of distance. This idea comes from the Anticipatory Scheduler (AS) about anticipating the location of the next request. This parameter allows the scheduler to anticipate requests in the “backward” or opposite direction and consider the requests as being “next” if they are within this distance from the current head location.
  2. back_seek_penalty
    This parameter is used to compute the cost of backward seeking. If the backward distance of a request is just (1/back_seek_penalty) from a “front” request, then the seeking cost of the two requests is considered equivalent and the scheduler will not bias toward one or the other (otherwise the scheduler will bias the selection to “front direction requests). Recall, the CFQ has the concept of elevators so it will try to seek in the current direction as much as possible to avoid the latency associated with a seek. This parameters defaults to 2 so if the distance is only 1/2 of the forward distance, CFQ will consider the backward request to be close enough to the current head location to be “close”. Therefore it will consider it as a forward request.
  3. fifo_expire_async
    This particular parameter is used to set the timeout of asynchronous requests. Recall that CFQ maintains a fifo (first-in, first-out) list to manage timeout requests. In addition, CFQ doesn’t check the expired requests from the fifo queue after one timeout is dispatched (i.e. there is a delay in processing the expired request). The default value for this parameter is 250 ms. A smaller value means the timeout is considered much more quickly than a larger value.
  4. fifo_expire_sync
    This parameter is the same as fifo_expire_async but for synchronous requests. The default value for this parameter is 125 ms. If you want to favor synchronous request over asynchronous requests, then this value should be decreased relative to fifo_expire_asynchronous.
  5. slice_sync
    Remember that when a queue is selected for execution, the queues IO requests are only executed for a certain amount of time (the time_slice) before switching to another queue. This parameter is used to calculate the time slice of the synchronous queue. The default value for this parameter is 100 ms, but this isn’t the true time slice. Rather the time slice is computed from the following: time_slice = slice_sync + (slice_sync / 5 * 4 – io_priority)). If you want the time slice for the synchronous queue to be longer (perhaps you have more synchronous operations), then increase the value of slice_sync.
  6. slice_async
    This parameter is the same as slice_sync but for the asynchronous queue. The default is 40 ms. Notice that synchronous operations are preferred over asynchronous operations.
  7. slice_asyn_rq
    This parameter is used to limit the dispatching of asynchronous requests to the device request-queue in queue’s slice time. This limits the number of asynchronous requests are executed (dispatched). The maximum number of requests that are allowed to be dispatched also depends upon the io priority. The equations for computing the maximum number of requests is, max_nr_requests = 2 * (slice_async_rq + slice_async_rq * (7 – io_priority)). The default for slice_async_rq is 2.
  8. slice_idle
    This parameter is the idle time for the synchronous queue only. In a queue’s time slice (the amount of time operations can be dispatched), when there are no requests in the synchronous queue CFQ will not switch to another queue but will sit idle to wait for the process creating more requests. If there are no new requests submitted within the idle time, then the queue will expire. The default value for this parameter is 8 ms. This parameters can control the amount of time the schedulers waits for synchronous requests. This can be important since synchronous requests tend to block execution of the process until the operation is completed. Consequently, the IO scheduler looks for synchronous requests within the idle window of time that might come from a streaming video application or something that needs synchronous operations.
  9. quantum
    This parameter controls the number of dispatched requests to the device queue, request-device (i.e. the number of requests that are executed or at least sent for execution). In a queue’s time slice, a request will not be dispatched if the number of requests in the device request-device exceeds this parameter. For the asynchronous queue, dispatching the requests is also restricted by the parameter slice_async_rq. The default for this parameter is 4.

You can see that the CFQ scheduler prefers synchronous IO requests. The reason for this is fairly simple – synchronous IO operations block execution. So until that IO operation is executed the application cannot continue to run. These applications can include streaming video or streaming audio (who wants their movie or music to be interrupted?), but there are a great deal more applications that perform synchronous IO.

On the other hand, Asynchronous IO (AIO) can be very useful because execution immediately returns to the application immediately without waiting for confirmation that the operation has completed. This allows the application to “overlap” computation and IO. This can be very useful for many operations depending upon the goals and requirements. There is quite a good article that talks about synchronous and asynchronous and blocking and non-blocking IO requests.

Quick Tuning Example

Comments on "Tuning CFQ – What Station is That?"

Major thankies for the blog post.Really looking forward to read more.

I just want to say I am all new to weblog and seriously liked you’re website. Probably I’m want to bookmark your site . You certainly come with fantastic article content. Bless you for sharing with us your web site.

I just want to mention I am just beginner to blogs and definitely liked you’re blog. Very likely I’m likely to bookmark your site . You definitely come with fantastic stories. Bless you for sharing your webpage.

Looking forward to reading more. Great blog post.Really looking forward to read more. Will read on

Perfect piece of work you have done, this web site is really cool with great info.

I can’t see half texts on this page after updating my browser.

Although websites we backlink to below are considerably not connected to ours, we really feel they may be in fact worth a go as a result of, so possess a look.

If some one wants expert view concerning blogging and site-building then i propose him/her to go to see this weblog, Keep up the nice work.

cialis venta cialis comprar cialis cialis sin receta cialis generique vente cialis generico cialis acquisto cialis cialis kaufen Cialis potenzmittel

Thanks-a-mundo for the post. Want more.

cialis cialis
precio cialis precio cialis
acheter cialis achat cialis
cialis acquistare cialis
cialis cialis

Im grateful for the blog post.Thanks Again.

Very neat blog article. Really Cool.

cialis cialis
cialis sin receta venta cialis
cialis pas nher achat cialis
acquistare cialis generico cialis
cialis generika cialis deutschland

I just want to say I am beginner to blogging and site-building and certainly enjoyed your web site. Most likely I’m planning to bookmark your blog post . You definitely have beneficial stories. With thanks for sharing your web-site.

precio cialis cialis generico
comprar cialis cialis sin receta
cialis sans ordonnance acheter cialis
comprare cialis acquistare cialis
cialis preise tadalafil

cialis generico comprar cialis
achat cialis cialis pas cher
propecia online order propecia
order kamagra purchase kamagra

Awsome site! I am loving it!! Will come back again. I am taking your feeds also

I simply want to say I’m beginner to blogs and definitely liked you’re web blog. Likely I’m going to bookmark your blog post . You certainly have awesome writings. With thanks for revealing your blog.

cialis generique vente cialis
cialis cialis generico
cialis acheter cialis
comprar cialis cialis generico
comprar cialis cialis

Hi there! Would you mind if I share your blog with my facebook group? There’s a lot of people that I think would really appreciate your content. Please let me know. Many thanks

vente cialis acheter cialis
cialis cialis
vente cialis acheter cialis
comprar cialis comprar cialis
precio cialis comprar cialis

We’re a group of volunteers and starting a new scheme in our community. Your site provided us with valuable info to work on. You have done an impressive job and our entire community will be thankful to you.

I truly appreciate this article post.Really looking forward to read more.

Here are some hyperlinks to web pages that we link to since we believe they’re really worth visiting.

I really like and appreciate your article post.Thanks Again. Awesome.

Read my latest blog post “Trying To Learn More On Search Engine Optimization? Check This Article Out! ” from ” http://massmarketing.pro/trying-to-learn-more-on-search-engine-optimization-check-this-article-out.html

If you like it, don’t forget to share.

Great post. Will read on…

I am so grateful for your article post.Much thanks again.

acheter cialis vente cialis
cialis acquistarecialis
cialis achat vente cialis
precio cailis venta cialis
cialis comprar cialis

Although internet websites we backlink to beneath are considerably not related to ours, we really feel they are basically really worth a go as a result of, so have a look.

Here are some hyperlinks to websites that we link to since we think they are really worth visiting.

Although internet websites we backlink to below are considerably not connected to ours, we really feel they may be actually worth a go via, so have a look.

Every when inside a whilst we select blogs that we read. Listed below would be the most up-to-date internet sites that we select.

Usually posts some really intriguing stuff like this. If you are new to this site.

Here are some of the web-sites we advocate for our visitors.

Here are a number of the websites we advise for our visitors.

We came across a cool web page that you may get pleasure from. Take a appear should you want.

Here are some links to websites that we link to since we feel they’re worth visiting.

Usually posts some incredibly interesting stuff like this. If you are new to this site.

That is the end of this post. Here you?ll come across some sites that we think you will appreciate, just click the links.

Here are some links to web-sites that we link to simply because we assume they are worth visiting.

Below you will uncover the link to some websites that we believe you should visit.

The facts mentioned within the post are several of the most effective out there.

Always a big fan of linking to bloggers that I enjoy but don?t get a lot of link really like from.

Here are some hyperlinks to internet sites that we link to since we think they’re worth visiting.

We like to honor a lot of other net web pages on the internet, even if they aren?t linked to us, by linking to them. Below are some webpages worth checking out.

Sites of interest we have a link to.

Leave a Reply