[gentoo-user] Re: aligning SSD partitions

[Nicolas Sebrecht <nsebrecht@piing.fr>]

The 06/09/12, Dale wrote:

> Then take a look at it this way.  If I emerge seamonkey with portage's
> work directory on disk and it takes 12 minutes, the first time.  Then I
> clear the caches and emerge seamonkey again while portage's work
> directory is on tmpfs and it is 12 minutes. Then repeat that process a
> few times more. If the outcome of all those emerges is 12 minutes,
> regardless of the order, then putting portages work directory on tmpfs
> makes no difference at all in that case.

We fully agree with you, here.

>                                           The emerge times are exactly
> the same regardless of emerge using cache or not or portage's work
> directory being on tmpfs or not.  I don't care if emerge uses cache
> DURING the emerge process because it is always enabled in both tests. 

But you *should* care. If you don't have enough memory, the kernel will
reclaim memory from the pagecache, so the whole process rapidity won't
only rely on RAM rapidity anymore.

> The point is whether portage's work directory is on tmpfs or not makes
> emerges faster.
> 
> The thing about what you are saying is that I ran those tests with the
> files in memory.  What I am saying is this, that is not the case.  I am
> clearing that memory with the drop_cache command between each test. You
> claim that cache is affecting the timing but I am clearing the very same
> cache the same as a reboot would. The emerge times whether portage's

We do agree with you that you droped the cache between the tests with
almost the same effect of a reboot.

>                                   The emerge times whether portage's
> work directory is on tmpfs or not didn't change enough to make a
> difference.

Yes, we agree. You droped the cache which is expected to get correct
tests.

What we are saying is that you droped the cache but did NOT DISABLED the
VM caches (kernel cache). You say that you don't care of that one
because it was involved in all the tests. We say that you might not care
in some contexts, not for all the contexts. You reach the context where
it does not matter much, fine.

-- 
Nicolas Sebrecht