On 01/28/2016 05:03 PM, Daniel Walker wrote:

On 01/28/2016 03:58 PM, Johannes Weiner wrote:

quoted

On Thu, Jan 28, 2016 at 03:42:53PM -0800, Daniel Walker wrote:

quoted

"Currently there is no way to figure out the droppable pagecache size
from the meminfo output. The MemFree size can shrink during normal
system operation, when some of the memory pages get cached and is
reflected in "Cached" field. Similarly for file operations some of
the buffer memory gets cached and it is reflected in "Buffers" field.
The kernel automatically reclaims all this cached & buffered memory,
when it is needed elsewhere on the system. The only way to manually
reclaim this memory is by writing 1 to /proc/sys/vm/drop_caches. "

[...]

quoted

The point of the whole exercise is to get a better idea of free 
memory for
our employer. Does it make sense to do this for computing free memory?

/proc/meminfo::MemAvailable was added for this purpose. See the doc
text in Documentation/filesystem/proc.txt.

It's an approximation, however, because this question is not easy to
answer. Pages might be in various states and uses that can make them
unreclaimable.

Khalid was telling me that our internal sources rejected MemAvailable 
because it was not accurate enough. It says in the description,
"The estimate takes into account that the system needs some page cache 
to function well". I suspect that's part of the inaccuracy. I asked 
Khalid to respond with more details on this.

Some quotes,

"
[regarding MemAvaiable]

This new metric purportedly helps usrespace assess available memory. But,
its again based on heuristic, it takes 1/2 of page cache as reclaimable..

Somewhat arbitrary choice. Maybe appropriate for desktops, where page
cache is mainly used as page cache, not as a first class store which is
the case on embedded systems. Our systems are swap less, they have little
secondary storage, they use in-memory databases/filesystems/shared memories/
etc. which are all setup on page caches).. This metric as it is implemented
in 3.14 leads to a totally mis-leading picture of available memory"

Daniel