Re: [gentoo-amd64] tmpfs help

[Volker Armin Hemmann]

On Mittwoch, 13. Februar 2008, Richard Freeman wrote:
> Volker Armin Hemmann wrote:
> > On Mittwoch, 13. Februar 2008, Richard Freeman wrote:
> >> Do you have benchmarks to support this?
> >
> > which numbers? that swap is horrible slow compared to ram?
>
> No - that compiling in tmpfs is horrible compared to compiling to disk.
>   Neither of us is proposing getting rid of disk access entirely - we're
> just debating two different ways of doing it.

tmpfs is fine - if you have enough ram. 1gb is not enough.



> >> As opposed to wasting all your RAM on cache&buffers to hold all those
> >> files being accessed?  During compilation that RAM is going to be
> >> heavily used - no getting around that.  Once you're done any files left
> >> sitting on a tmpfs will just get paged out until accessed.  They
> >> shouldn't really use any RAM at all.  Even if those files were on disk
> >> they would consume RAM in the form of caching until they're considered
> >> unneeded.
> >
> > cached&buffered information can be discarded anytime if the ram is needed
> > elsewhere. tmpfs has to be shoved into swap.
>
> Uh, data written to a file has to be written to disk before the
> cache/buffer is flushed.  The difference is that the data is written to
> disk within about 10 seconds of being written to the buffer, and then
> retained in cache maybe for a few hours longer (optimistically).  With
> tmpfs the write doesn't happen until the space is needed.  Either way
> every byte gets written - with tmpfs it is deferred as late as possible
> but with disk-based filesystems the write is done as early as possible.
>

emm, no.


> > Extreme example?
> >
> > kdepim with the enablefinal flag. On amd64 every single gcc instance
> > needs ~900mb at two points of compilation. With 1gb ram and no tmpfs it
> > sucks.
> >
> > With 1gb ram and 512mb of that reserved for tmpfs, you'll get a
> > swapstorm.
>
> Oh, it will certainly swap a great deal.  However, I don't see why it
> would be any slower - either way you're doing a ton of disk access.  Do
> you have actual benchmarks?

Not real benchmarks. But kdepim with enablefinal, 1gb of ram and -j2 several 
hours. With j1 2h.
.
kdepim with 2gb of ram and j2 30m
Just because first case swap storn, last case no swap at all.

>
> Again, tmpfs doesn't "reserve" memory - it uses memory - just like
> cache/buffers.

but while cache/buffers can be discarded when the ram is needed, tmpfs has to 
be shoved into butt-slow swap.

>
> >> I certainly agree that
> >> swap is slow compared to RAM, but it isn't slow compared to a disk-based
> >> filesystem.

yes, yes it is. It is faster to start an app from disk, then to fetch it out 
of swap. My very personal experience since many many years.


> >
> > really? Every really swapped? IMHO it feels like every single byte is
> > fetched by a mule.
>
> Right now I've got 589MB of free RAM (-/+ buffers/cache) with 2GB of RAM
> total.  I've got 705MB of swap used.  Works just fine IMHO.  Sure, if I
> make it really busy it can get slow, although with nice/ionice there
> isn't much visible

your system would feel and act a lot faster if you don't have anything in 
swap. 'Fine' is good - as long as you don't know the alternative.

> >  Of course adding more RAM will never hurt, but that incurs
> >
> >> significant cost and you can at least maximize your current hardware
> >> before investing in more of it.
> >
> > significant costs like 30euros for 2gb?
>
> Sure.  Compared to about $1 for 2GB of hard drive space 30 euros is
> significant.

don't forget that ram is also roughly 100 times faster than harddisk.

> And even if he had 2GB of RAM I submit that it would
> probably work better if he compiled in tmpfs than on disk.

with 2gb he could do it. But with 1gb he just hurts himself.


> There is no
> question that a $550 computer will outperform a $500 computer, and a
> $600 computer will outperform a $550 computer, and so on.  You can
> always spend 30 more euros and improve your system.  What I'm interested
> in is maximizing the performance of the system I have now.  I can always
> spend $50 and make it even faster.  However, there are lots of things I
> can spend $50 on - I'd rather spend it on something else all things
> being equal.

you could stop shoving everything in swap - no costs involved and system is a 
lot faster.


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