Re: free: regression due to a different calculation of Used memory

OK to see what's going on I've done some testing.

The first thing I'd like to say is that testing this is tricky, the first
set gave some terrible results for the proposed change because the free and
cached value changed dramatically (~ 40M) during the test.
It seemed that cached was low and increased once you start doing things and
remains high.

On Wed, 1 Jun 2022 at 19:50, Jan Rybar <[email protected]> wrote:

> > You won't see that in meminfo.c or vmstat.c, but Shmem page belong to
> the page cache and as such are accounted in cached.
> > If you write a big file in tmpfs, you'll see that both shmem and cache
> entry increase
>
I've done that very test.
I added a 5MB file to a tmpfs directory twice, taking memory samples each
time.

Free /proc/meminfo
State Total Used Free Shared Buff/Cache Available Free Cached SReclaimable
Buffers Used: Original Used: No cache Used: Availabe Used: Simple
No file 976.1 83.5 744.8 4.5 147.9 753.7 744.6 114 19.8 14.1 83.6 197.6
222.4 231.5
First 5M file 976.1 83.4 739.9 9.5 152.9 748.8 739.9 119 19.8 14.1 83.3
202.3 227.3 236.2
Delta from init 0 -0.1 -4.9 5 5 -4.9 -4.7 5 0 0 -0.3 4.7 4.9 4.7
Second 5M file 976.1 83.5 734.7 14.5 157.9 743.7 734.5 124 19.8 14.2 83.6
207.6 232.4 241.6
Delta from init 0 0 -10.1 10 10 -10 -10.1 10 0 0.1 0 10 10 10.1
Remove files 976.1 83.3 744.8 4.5 148 753.8 744.8 114 19.8 14.2 83.3 197.3
222.3 231.3
Delta from init 0 -0.2 0 0 0.1 0.1 0.2 0 0 0.1 -0.3 -0.3 -0.1 -0.2

Adding the first file, we can use the current method for Used shows a
*reduction* of 300k for used and no change for the second. Removing Cached
or a simple Total-Free shows a reduction of 4.7 MB. Total-Available is 4.9
With the second 5MB file, the current method shows no change but the other
three methods we can see Used increasing by 10 or 10.1 MB.

I think we can safely say putting Cached into the Used calculation is
wrong. There is definitely 5 or 10MB of memory that is being used and the
current free won't show this. It's probably a failure of Cached giving the
"right" answer rather than free's fault, but we are beholden to what the
kernel gives us. Cached used to mean cache for files paged/swapped to disk
but it doesn't anymore.

I also explored doing some delta of (Cached-Shmem) but for some systems
they both increased by 5MB which was good, but for others Cached increased
by 6MB and Shmem by 5MB.

We have several options here:

   1. Original: Used = Total - Free - Cached - SReclaimable - Buffers
   2. No Cache: Used = Total - Free - SReclaimable - Buffers
   3. Available: Used = Total - Available
   4. Simple: Used = Total - Free

I think we can all agree that at least for files going into a tmpfs the
original formula is wrong. We definitely are using an extra 5 or 10MB here
but free won't show it.
Methods 2 and 4 can be accused of missing something or having the wrong
definition of what Used should be ("you forgot to include X value")

My preferred option is to say that Used memory is unavailable memory, in
other words option 3 "Used =Total - Available".  The kernel team is
responsible for the Available value, we are just reporting a simple
difference between the two.  Remember that the used calculation predates
the creation of the Available value which is why we have had various
calculations for what Used is. Option 3 takes this away from procps which
is my preferred option.

If we can get a consensus we can put this to bed, so comments are welcome.

 - Craig