[CentOS] Errors on an SSD drive

Thu Aug 10 22:10:28 UTC 2017
Warren Young <warren at etr-usa.com>

On Aug 10, 2017, at 2:17 PM, John R Pierce <pierce at hogranch.com> wrote:
> On 8/10/2017 1:12 PM, Warren Young wrote:
>> You want those pages to get swapped out quickly so that the precious RAM can be used more productively; by the buffer cache, if nothing else.
> most modern virtual memory OS's don't swap out unused pages, instead, they swap IN accessed pages directly from the executable file.  only thing written to swap are 'dirty' pages that have been changed since loading.

Is that not a distinction without a difference in my case?

Let’s say I have a system with 256 MB of free user-space RAM, and I have a binary that happens to be nearly 256 MB on disk, between the main executable and all the libraries it uses.

Question: Can my program allocate any dynamic RAM?

The OS’s VMM is free to use addresses beyond 0-256 MB, but since we’ve said there is no swap space, everything swapped in must still be assigned a place in physical RAM *somewhere*.

Is there a meaningful distinction between:

Scenario 1: The application’s first few executable pages are loaded from disk, a few key libraries are loaded, then the application does a dynamic memory allocation, then somehow causes all the rest of the executable pages to be loaded, running the system out of RAM.

Scenario 2: The application is entirely loaded into RAM, nearly filling it, then the application attempts a large dynamic memory allocation, causing an OOM error.

Regardless of the answer to these questions, I can tell you that switching that web site to a more efficient web application stack allowed us to shrink the VPS from a 256 MB plan, under which it would occasionally crash and require a reboot, to a 64 MB plan, under which the site has been rock-solid.  Same VPS provider, same web site content, same user-facing functionality.

If I’d had the ability to assign swap space, I probably could have gotten away with a 64 MB VPS plan with the inefficient web technology, too.  They gave me plenty of disk space with that plan.

(And no, swapon /some-file is no solution here.  The VPS technology simply didn’t allow swap space, even from a swap file on one of the system disks.  It wasn’t simply an inability to add a swap partition.)