throttle cpu usage for a cronjob

Question

I've got a cronjob that is basically nothing more than mysqldump whatever | gzip > backup.sql.gz. The database is fairly big. I've noticed that while the job is running, my apache is much less responsive, the CPU usage is like this

 %CPU  COMMAND 
 15.1  gzip               
 14.1  mysqld             
 13.1  mysqldump 

Is there a way to make this job use less CPU, maybe at the price of it taking longer?

Answer 1

 ionice -c 3 nice mysqldump whatever | ionice -c 3 nice gzip > backup.sql.gz

Answer 2

If your web app is writing to the 'whatever' database, then it has to wait for mysqldump to complete, because it locks tables by default. If this is the case and you lower the priority, you'll make the problem worse.

Dump the database first as plain text, then run gzip to compress the file. This way mysqldump can complete sooner and release the lock, and MySQL and Apache can go about their business while gzip runs.

Answer 3

You can use nice and renice to change the priority of processes. This will mean they get less access to the CPU.

You probably don't want to renice mysqld but mysqldump and gzip can safely have their priorities changed.

If you're having trouble with too much disk usage, you can use ionice.