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Help, Linux ate my RAM

linuxatemyram.com

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Re: Help, Linux ate my RAM

#21

Post by plaes »

mjb wrote: post #13
plaes wrote: post #10

The cache can be cleared via `/proc/sys/vm/drop_caches`. http://linux-mm.org/Drop_Caches

Yes it can, but you probably don't want to do that. You definitely don't want to do it in an automated way when the machine is experiencing memory pressure. There are very good reasons that Linux (and most other modern operating systems) makes aggressive use of page caches and buffers. For the vast majority of applications dropping these caches is going to reduce performance considerably (disk is really really slow)…

Yes, it was actually sort of a response to that webpage that said it was not possible to free this cached memory.

The cache dropping is actually useful when you are doing benchmarking...

Re: Help, Linux ate my RAM

#23

Post by obtu »

bshep wrote: post #2

Is there any way to have top display this information?

free does the job of taking cache into account (mentioned in the original post). If you've read the neugierig post, and want a better per-process monitor:

gnome-system-monitor has a top-like monitor as well as graphs, and measures memory properly (including a discount for shared maps); smem works in the console; it doesn't have a term interface like top, but it can be combined with watch.

Re: Help, Linux ate my RAM

#24

Post by francoisdevlin »

krobertson wrote: post #19
illumin8 wrote: post #11

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 environmen…

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.

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!

Re: Help, Linux ate my RAM

#25

Post by ajross »

krobertson wrote: post #19
illumin8 wrote: post #11

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 environmen…

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.

That's depressing. Developers are the ones who have to understand this. In IT it's common to find people (even "DBAs") who have made a career of following procedures someone else wrote.

Re: Help, Linux ate my RAM

#27

Post by mjb »

plaes wrote: post #21
mjb wrote: post #13

Earlier quoted context omitted.

Yes it can, but you probably don't want to do that. You definitely don't want to do it in an automated way when the machine is experiencing memory pressure. There are very good reasons that Linux (and most other modern operating systems) makes aggressive use of page caches and buffers. For the vast majority of applications dropping these caches is going to reduce performance considerably (disk is really really slow)…

Yes, it was actually sort of a response to that webpage that said it was not possible to free this cached memory. The cache dropping is actually useful when you are doing benchmarking...

> The cache dropping is actually useful when you are doing benchmarking...

Agreed.

My response was more to the "Let's Put 'echo 3 > /proc/sys/vm/drop_caches' In Cron and Get Free RAM!!!!" thinking, which sadly seems to be widespread.

Re: Help, Linux ate my RAM

#30

Post by ivan78 »

francoisdevlin wrote: post #24
krobertson wrote: post #19

Earlier quoted context omitted.

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.

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!

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.
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