[lug] stupid swap question

[Steve Litt]

On Sun, 1 Dec 2019 13:29:54 -0700
Bear Giles <bgiles at coyotesong.com> wrote:

> I have a stupid swap question....

I'm going to answer a different question, just in case my answer helps.

> 
> My system has 64 GB of memory, or nearly enough for 3 pages on Chrome.
> 
> Obviously I want to remain memory-resdent as much as possible but I
> would also like a soft landing when Chrome does what Chrome does so
> well - I would rather have some background stuff move to swap than to
> have to do a hard reset because the system locked up.

Me too. Chromium is such a pig I went back to firefox, which of course
is a slightly smaller pig.

I made tremendous strides on my system by hugely reducing the number of
files Firefox could keep in cache. Firefox was keeping tens of
thousands of cache files in a single directory. At least with Ext4,
Linux is horrible at finding one file in a directory of 20,000. Like
seconds per file. A better design would have been to have 26
directories, a to z, each with subdirectories, a to z, maybe to four or
five levels, and put the actual files in the leaf directories. Linux is
lightning quick finding one out of 26. But that's crying over spilled
milk.

So after seeing tens of thousands of cache files in one directory, and
knowing that would make what should have been a millisecond retrieval
into three seconds, I decided that if I hadn't accessed a resource
within the past hour, it would be better to just download it again. I
don't remember how I did it, it's very obscure, but I limited the
maximum number of cache files to a pretty low 3 or 4 digit number. The
effect on Firefox and all the programs running at the same time was
stunning.

> 
> I've created an explicit swap file, added it via swapon, and it shows
> up in the system but doesn't seem to be used. It's really odd since
> this has worked in the past.

Somebody else pointed out that swap memory is a circuit breaker: It
doesn't happen until needed. To really test it you'd need to open
Chromium with a bunch of gratuitously scripted websites until it choked.

Keep in mind it might not be choking on RAM at all. It could be disk or
CPU. One thing I'd try is to have a crm.sh that runs chromium with a
nice level of 19. 

nice -n 39 chromium

I used 39 because -n is an adjustment, not an absolute number, and if
the previous niceness had been -20, it would take 39 to get it up to
the max of 19.

If the problem is CPU, nicing up chromium should enable you to
manipulate other programs long enough to send a HUP to chromium.
 
SteveT

Steve Litt
November 2019 featured book: Manager's Guide to Technical
Troubleshooting Second edition
http://www.troubleshooters.com/mgr