thrashing on file pages

[Luigi Semenzato <semenzato@google.com>]

Greetings MM community, and apologies for being out of touch.

We're running into a MM problem which we encountered in the early
versions of Chrome OS, about 7 years ago, which is that under certain
interactive loads we thrash on executable pages.

At the time, Mandeep Baines solved this problem by introducing a
min_filelist_kbytes parameter, which simply stops the scanning of the
file list whenever the number of pages in it is below that threshold.
This works surprisingly well for Chrome OS because the Chrome browser
has a known text size and is the only large user program.
Additionally we use Feedback-Directed Optimization to keep the hot
code together in the same pages.

But given that Chromebooks can run Android apps, the picture is
changing.  We can bump min_filelist_kbytes, but we no longer have an
upper bound for the working set of a workflow which cycles through
multiple Android apps.  Tab/app switching is more natural and
therefore more frequent on laptops than it is on phones, and it puts a
bigger strain on the MM.

I should mention that we manage memory also by OOM-killing Android
apps and discarding Chrome tabs before the system runs our of memory.
We also reassign kernel-OOM-kill priorities for the cases in which our
user-level killing code isn't quick enough.

In our attempts to avoid the thrashing, we played around with
swappiness.  Dmitry Torokhov (three desks down from mine) suggested
shifting the upper bound of 100 to 200, which makes sense because we
use zram to reclaim anonymous pages, and paging back from zram is a
lot faster than reading from SSD.  So I have played around with
swappiness up to 190 but I can still reproduce the thrashing.  I have
noticed this code in vmscan.c:

        if (!sc->priority && swappiness) {
                scan_balance = SCAN_EQUAL;
                goto out;
        }

which suggests that under heavy pressure, swappiness is ignored.  I
removed this code, but that didn't help either.  I am not fully
convinced that my experiments are fully repeatable (quite the
opposite), and there may be variations in the point at which thrashing
starts, but the bottom line is that it still starts.

Are we the only ones with this problem?  It's possible, since Android
by design can be aggressive in killing processes, and conversely
Chrome OS is popular in the low-end of the market, where devices with
2GB of RAM are still common, and memory exhaustion can be reached
pretty easily.  I noticed that vmscan.c has code which tries to
protect pages with the VM_EXEC flag from premature eviction, so the
problem might have been seen before in some form.

I'll be grateful for any suggestion, advice, or other information.  Thanks!

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