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Re: /tmp as tmpfs and consequence for imaging software

On Sat, Nov 12, 2011 at 11:25 PM, Josselin Mouette <joss@debian.org> wrote:
> Le samedi 12 novembre 2011 à 23:12 +0100, Samuel Thibault a écrit :
>> Adam Borowski, le Sat 12 Nov 2011 23:08:08 +0100, a écrit :
>> > You need to increase the swap size by the amount you'd use for /tmp.
>> Well, the idea of such case is precisely to *not* use swap, but real
>> disks. Such software already know how to manage its memory and
>> disk-backed memory (thusly stored in /tmp)
> Practically speaking, the only significant difference is that files are
> not forced to disk as early. Otherwise, if you have a large enough swap,
> pages of a file on a tmpfs that are not used enough will be swapped. And
> pages of a file on a regular filesystem that are used enough will be
> kept in the buffer cache.

No it is not true. Science and imaging software are better to use true
disk baked file. For instance, if I want ot invert a big matrix they
are pretty good algorithm that force only some part of the file to be
keep on disk. They known better than kernel when to put somepart on
the data on the slow disk.

Using tmpfs under /tmpfs you break assumptions on the life expendancy
of memory object. And you slow down this kind of software (that work
perfecly for 40 years).

> OTOH, for a wide range of applications that do a lot of small writes,
> using tmpfs is a huge gain.

Yes for desktop. Not for software that know ho to use backed disk file.

And do not ask user to choose. Imageging software and movie maker
software have the same requirement than high performance science
They need file backed.

Now that are the solution ? We could not increase tmpfs over 50% to
70% of physical ram without deadlock (OOM and so on). And it is not
enought for your use. Should we use /var/tmp  ? But it does not fit
due to "Files and directories located in /var/tmp must not be deleted
when the system is booted" (FHS). Whereas this kind of software want
to be non persistant and tru file backed.

Any suggestion is welcome

> -
>  .''`.      Josselin Mouette
> : :' :
> `. `'
>  `-

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