linux-kernel - RE: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

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Date:	Sun, 2 May 2010 08:05:29 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Avi Kivity <avi@...hat.com>, Jeremy Fitzhardinge <jeremy@...p.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	hugh.dickins@...cali.co.uk, ngupta@...are.org, JBeulich@...ell.com,
	chris.mason@...cle.com, kurt.hackel@...cle.com,
	dave.mccracken@...cle.com, npiggin@...e.de,
	akpm@...ux-foundation.org, riel@...hat.com
Subject: RE: Frontswap [PATCH 0/4] (was Transcendent Memory): overview

> > So there are two users of frontswap for which the synchronous
> > interface makes sense.  I believe there may be more in the
> > future and you disagree but, as Jeremy said, "a general Linux
> > principle is not to overdesign interfaces for hypothetical users,
> > only for real needs."  We have demonstrated there is a need
> > with at least two users so the debate is only whether the
> > number of users is two or more than two.
> >
> > Frontswap is a very non-invasive patch and is very cleanly
> > layered so that if it is not in the presence of either of
> > the intended "users", it can be turned off in many different
> > ways with zero overhead (CONFIG'ed off) or extremely small overhead
> > (frontswap_ops is never set; or frontswap_ops is set but the
> > underlying hypervisor doesn't support it so frontswap_poolid
> > never gets set).
> 
> Yet there are less invasive solutions available, like 'add trim
> operation to swap_ops'.

As Nitin pointed out much earlier in this thread:

"No: trim or discard is not useful"

I also think that trim does not do anything for the widely
varying dynamically changing size that frontswap provides.
 
> So what needs to be said here is 'frontswap is XX times faster than
> swap_ops based solution on workload YY'.

Are you asking me to demonstrate that swap-to-hypervisor-RAM is
faster than swap-to-disk?

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