Skip to main content

Crontab



Cron is the task scheduler in linux based system. Any user can add their job by editing crontab. The job runs at the time specified by the user.
To add a job
1)type crontab -e in the shell
2)Specify hour, minute, date of month, month, day of the week (as required)
3)Specify the command (Eg. echo "happi Bday"|sendmail "kalyanceg@eskratch.com"
4)save with :wq and exit
The cron will get the job done for you at the time scheduled by you

Algo behind Cron
Initially cron sleeps for 1 minute and then checks for any process in the queue. If so, does the job and then sleeps for 1 min. This naive method is not scaleable.

Then the jobs are put in event list. The head will be the recent most process to be completed in the future. The cron sleeps till the head is scheduled. Once it discharges the head, its sleeping time becomes (next head's time-now time). It cant sleep without taking into account new entries. So a SIGHUP interrupt will wake the sleeping cron tab to readjust its sleeping time once a new job is added.

Comments

Post a Comment

Popular posts from this blog

How we have systematically improved the roads our packets travel to help data imports and exports flourish

This blog post is an account of how we have toiled over the years to improve the throughput of our interDC tunnels. I joined this company around 2012. We were scaling aggressively then. We quickly expanded to 4 DCs with a mixture of AWS and colocation. Our primary DC is connected to all these new DCs via IPSEC tunnels established from SRX. The SRX model we had, had an IPSEC throughput of 350Mbps. Around December 2015 we saturated the SRX. Buying SRX was an option on the table. Buying one with 2Gbps throughput would have cut the story short. The tech team didn't see it happening. I don't have an answer to the question, "Is it worth spending time in solving a problem if a solution is already available out of box?" This project helped us in improving our critical thinking and in experiencing the theoretical network fundamentals on live traffic, but also caused us quite a bit of fatigue due to management overhead. Cutting short the philosophy, lets jump to the story.
8 comments
Read more

LXC and Host Crashes

 We had set up a bunch of lxc containers on two servers each with 16 core CPUs and 64 GB RAM(for reliability and loadbalancing). Both the servers are on same vlan. The servers need to have atleast one of their network interface in promiscuous mode so that it forwards all packets on vlan to the bridge( http://blogs.eskratch.com/2012/10/create-your-own-vms-i.html ) which takes care of the routing to containers. If the packets are not addressed to the containers, the bridge drops the packet. Having this setup, we moved all our platform maintenance services to these containers. They are fault tolerant as we used two host machines where each host machine has a replica of the containers on the other. The probability to crash for both the servers at the same time due to some hardware/software failure is less. But to my surprise both the servers are crashing exactly the same time with a mean life time 20 days. We had to wake up late nights(early mornings) to fix stuffs that gone down The
Post a Comment
Read more

The FB outage

 This outage has caused considerable noise everywhere. It was quite discomforting for me because during the whole conversation nobody bothered to understand the gravity of the issue. I don't expect end users to understand the issue. But this is going to be a blogpost for all of those in the tech field, Such an event can happen how much ever chaos engineering, best of the tech jargon we implement in the stack To all my Site Reliability Engineer friends, Site Up is our first priority. I myself said many a times outage is news and SREs should prevent outage. But I'm afraid this is leading to a cult in the industry who despises outages and takes no learnings from it. I don't know what has happened in Facebook. I can explain a scenario which may or may not be right but that can definitely show the gravity of the issue. Let's draw a probable Facebook architecture Disclaimer I don't work at Facebook. So this might not be how facebook routes traffic. This is based on my exp
2 comments
Read more