Re: [gentoo-user] tmp on tmpfs

[Rich Freeman <rich0@gentoo.org>]

On Wed, May 24, 2017 at 1:16 AM, Ian Zimmerman <itz@primate.net> wrote:
>
> I have long been in the camp that thinks tmpfs for /tmp has no
> advantages (and may have disadvantages) over a normal filesystem like
> ext3, because the files there are normally so small that they will stay
> in the page cache 100% of the time.
>

The file being in the page cache only speeds up reads of the file.  On
a conventional filesystem the file will still be forced to be
committed to disk within 30 seconds, or whatever you've set your max
writeback delay to.  That means guaranteed disk write IO.  If the
drive is mostly idle it will have no impact on performance, but if the
disk is fairly busy then it will, especially for spinning disks.  For
an SSD /tmp would be a source of erase cycles (which also have
performance implications, but there it is more of a wear issue).  When
the file is removed that would also generate write IO.

The flip side is that on most systems /tmp probably doesn't get THAT much IO.

On Gentoo doing your builds in tmpfs definitely has a large
performance impact, because there are a lot of files created during
the build process that are sizable but which don't end up getting
installed (object files mostly).  Plus you have the extraction of the
source itself.  For a typical build that is many MB of data being
extracted and then deleted after maybe a minute, which is a lot of
useless IO, especially when the actual install is probably creating a
fairly sizable IO queue on its own.

To avoid a reply, I'll also note that tmpfs does NOT require swap to
work.  It does of course require plenty of memory, and as with any
situation where lots of memory is required swap may be useful, but it
is not a requirement.

Others have mentioned zram.  I've used it, but unless something has
changed one of its limitations is that it can't give up memory.  That
is less of an issue if you're using swap since it can be swapped out
if idle.  However, if you're not using swap then you're potentially
giving up a chunk of RAM to do it, though less RAM than a tmpfs if it
is full most of the time (which I doubt is typically the case).

-- 
Rich