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I've tried to educate Oracle DBAs on why top is wrong and their memory really isn't being used. It's painful and they often refuse to believe that I know what

I've tried to educate Oracle DBAs on why top is wrong and their memory really isn't being used. It's painful and they often refuse to believe that I know what I'm talking about, and that they should use the free -m command to see what memory is actually available for use. Is there any particular reason why Oracle DBAs are less likely to believe this? Perhaps it's because most of them grew up in legacy UNIX environments rather than Linux.

greedo 15y ago
I've had the same discussion with Websphere administrators who can't grasp the concept of caching and the role of swapping. I even had one admin think that modifying the swappiness setting to keep memory free would be a good idea...
ivan78 15y ago
As a longtime Linux user/admin and beginning Oracle DBA I now know that all memory should be occupied by Oracle, not by OS. :-) Serious mode on: I'm sure it's a big problem to be narrow expert. They can be brilliant specialist in their field, but one step aside and they are absolutely helpless.
- illumin8 15y ago
  Thanks for the great comment. I really wish more people like yourself at least understood Linux memory management. I think it will truly help you to be an exceptional DBA. I can also understand how an expert DBA might not know or care too much about the OS underneath his software, although I would argue that if you understand the OS fundamentals, you will be that much better at whatever specialty you have.
  - ivan78 15y ago
    Thank you. I'm on my way to become exceptional DBA. Oracle is only 2 years younger than me, but unfortunately it has been developing much faster than I am. It literally takes years just to become familiar with all these layers of Oracle technologies. So, it's really hard to blame DBAs for not knowing how underlying OS works. :-)
krobertson 15y ago
I think this is a pretty common misconception overall. Working at startups, often find devs with multiple hats sometimes doing ops tasks. Seen many who hop on a system trying to diagnose some issue, fire up top and proclaim "OMG, the problem is we're running out of memory!" 2nd is explaining virtual/resident set size.
- francoisdevlin 15y ago
  Okay, I'm one of those devs. What is this virtual/resident set size thing you're talking about? EDIT: Thank you for all the helpful responses!
  - ivan78 15y ago
    Well, if you don't understand it, you must read some basic book about modern computer architecture. Short answer: It is possible to reserve memory address space without assigning actual physical memory. As your program runs it can dynamically assign physical memory pages to its address space when it needs it. Again - you must read a book if you want to understand it.
    - ivan78 15y ago
      Why downvoting? It may be not the best explanation, but IMO the closest to the way a "developer" should understand it. On Linux a virtual memory is committed with mmap call. Check this for details: http://stackoverflow.com/questions/2782628/any-way-to-reserve-but-not-commit-memory-in-linux http://stackoverflow.com/questions/2782628/any-way-to-reserv... This is basically how shared libraries and other shared objects are attached to your process.
      - henrikschroder 15y ago
        > Why downvoting? > you must read a book if you want to understand it. "Go read a book" is not a very constructive or helpful answer to a question that - as it turns out - could be answered with a brief comment.
gaius 15y ago
I've been an Oracle DBA for 15 years, and no-one uses top for that, as it's well known not to account in any sort of meaningful way for the way Oracle uses shared memory. The only thing it's useful for is seeing which of the sysadmin's Perl scripts is chewing the CPU.
seclorum 15y ago
Teach them to interpret and understand cat /proc/meminfo .. one of the most interesting things you can do with this is pipe it into gnuplot and watch it over time as things happen. Try it sometimes .. you might get through to one or two.
- ivan78 15y ago
  If you want to be more enterprisey, you can pipe it to SNMP counter and then draw a graph with your Network Monitoring System. It is much more convenient if you have more that few servers.
  - seclorum 15y ago
    Maybe that's more 'enterprisey', but its more 'system administrator'-ey to just use an onboard gnuplot and look at real graphs without putting much SNMP/traffic-load on the system of interest .. the purpose is to educate the user on how the system works, not use it as a monitoring device for lazy sysadmins.

greedo 15y ago

I've had the same discussion with Websphere administrators who can't grasp the concept of caching and the role of swapping. I even had one admin think that modifying the swappiness setting to keep memory free would be a good idea...

ivan78 15y ago

As a longtime Linux user/admin and beginning Oracle DBA I now know that all memory should be occupied by Oracle, not by OS. :-) Serious mode on: I'm sure it's a big problem to be narrow expert. They can be brilliant specialist in their field, but one step aside and they are absolutely helpless.

illumin8 15y ago
Thanks for the great comment. I really wish more people like yourself at least understood Linux memory management. I think it will truly help you to be an exceptional DBA. I can also understand how an expert DBA might not know or care too much about the OS underneath his software, although I would argue that if you understand the OS fundamentals, you will be that much better at whatever specialty you have.
- ivan78 15y ago
  Thank you. I'm on my way to become exceptional DBA. Oracle is only 2 years younger than me, but unfortunately it has been developing much faster than I am. It literally takes years just to become familiar with all these layers of Oracle technologies. So, it's really hard to blame DBAs for not knowing how underlying OS works. :-)

krobertson 15y ago

I think this is a pretty common misconception overall. Working at startups, often find devs with multiple hats sometimes doing ops tasks. Seen many who hop on a system trying to diagnose some issue, fire up top and proclaim "OMG, the problem is we're running out of memory!" 2nd is explaining virtual/resident set size.

francoisdevlin 15y ago
Okay, I'm one of those devs. What is this virtual/resident set size thing you're talking about? EDIT: Thank you for all the helpful responses!
- ivan78 15y ago
  Well, if you don't understand it, you must read some basic book about modern computer architecture. Short answer: It is possible to reserve memory address space without assigning actual physical memory. As your program runs it can dynamically assign physical memory pages to its address space when it needs it. Again - you must read a book if you want to understand it.
  - ivan78 15y ago
    Why downvoting? It may be not the best explanation, but IMO the closest to the way a "developer" should understand it. On Linux a virtual memory is committed with mmap call. Check this for details: http://stackoverflow.com/questions/2782628/any-way-to-reserve-but-not-commit-memory-in-linux http://stackoverflow.com/questions/2782628/any-way-to-reserv... This is basically how shared libraries and other shared objects are attached to your process.
    - henrikschroder 15y ago
      > Why downvoting? > you must read a book if you want to understand it. "Go read a book" is not a very constructive or helpful answer to a question that - as it turns out - could be answered with a brief comment.

gaius 15y ago

I've been an Oracle DBA for 15 years, and no-one uses top for that, as it's well known not to account in any sort of meaningful way for the way Oracle uses shared memory. The only thing it's useful for is seeing which of the sysadmin's Perl scripts is chewing the CPU.

seclorum 15y ago

Teach them to interpret and understand cat /proc/meminfo .. one of the most interesting things you can do with this is pipe it into gnuplot and watch it over time as things happen. Try it sometimes .. you might get through to one or two.

ivan78 15y ago
If you want to be more enterprisey, you can pipe it to SNMP counter and then draw a graph with your Network Monitoring System. It is much more convenient if you have more that few servers.
- seclorum 15y ago
  Maybe that's more 'enterprisey', but its more 'system administrator'-ey to just use an onboard gnuplot and look at real graphs without putting much SNMP/traffic-load on the system of interest .. the purpose is to educate the user on how the system works, not use it as a monitoring device for lazy sysadmins.