[gentoo-amd64] Re: Emerge and Tmpfs

[Duncan]

Frank Peters <frank.peters@×××××××.net> posted
20090708095104.39a8a53e.frank.peters@×××××××.net, excerpted below, on 
Wed, 08 Jul 2009 09:51:04 -0400:

> However, I am not clear on how tmpfs will fail.  If the tmpfs mount
> becomes filled or exceeds the file limit, since it is essentially just
> another disk partition shouldn't it produce a "No more space left on
> device" error?  Or is the system designed to extend the tmpfs through
> swapping? The latter option doesn't seem right, but everyone still
> refers to such behavior.

tmpfs is specifically /not/ pinned in real-memory, and only gets the "no 
space left on device" error when it maxes out.  (Note that the tmpfs 
default max side if not specified is half of real memory, the default max 
inodes are half of the memory page-count, which @ 4KB pages, means 256/MB 
or 262,144/gig.)

So yes, tmpfs swaps just fine, because it's not pinned memory and running 
out of real memory is not no-space-on-device, as long as there's swap.

For purposes of memory tracking, BTW, (unswapped) tmpfs is reported as 
cache, tho in some ways it really behaves more like (dirty) app memory in 
that it can't be simply dropped as the disk-read-cache can (since for 
cache the data is already on disk and can simply be read back from there 
if it's needed), but must be actually written out to swap, like dirty app 
memory.

But regardless, pointing portage's working dir at tmpfs is still 
generally faster than having it on a normal disk filesystem, because the 
worst-case for tmpfs, is the normal-case for a normal disk filesystem -- 
that it actually has to write the stuff to disk for a bit, before reading 
it back in and ultimately deleting it.

Of course, the same basic effect could be had by pointing portage at an 
ext4fs, configured with huge (several gig) max write buffers and long 
(like an hour) sync timeouts.  Except even then ext4 wouldn't be quite as 
good, because there's shortcuts the kernel can take with the explicitly 
temporary data of a tmpfs that it can't take with ext4.  That's the 
theory, anyway.  It'd be interesting to benchmark it.  Oh, and if the 
system /does/ crash in the middle of something, with tmpfs it's all 
"magically" cleaned up, while with ext4, whatever files had been written 
to disk would have to be cleaned up after the reboot.  So even if the 
benchmarks don't quite live up to the theory, avoiding that cleanup 
hassle is very likely worth it.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman