Subject: Re: [CentOS] Errors on an SSD drive



On Aug 10, 2017, at 2:17 PM, John R Pierce <[email protected]> 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
provi...

der, 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.)
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