Date: Mon, 8 Mar 2004 13:02:31 -0800
From: Andrew Morton <akpm@osdl.org>
To: Andrea Arcangeli <andrea@suse.de>
Cc: torvalds@osdl.org, linux-kernel@vger.kernel.org
Subject: Re: objrmap-core-1 (rmap removal for file mappings to avoid 4:4 in
 <=16G machines)
Message-Id: <20040308130231.59deef80.akpm@osdl.org>
In-Reply-To: <20040308202433.GA12612@dualathlon.random>
References: <20040308202433.GA12612@dualathlon.random>
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Andrea Arcangeli <andrea@suse.de> wrote:
>
> without this patch not even the 4:4 tlb overhead would allow intensive
> shm (shmfs+IPC) workloads to surivive on 32bit archs. Basically without
> this fix it's like 2.6 is running w/o pte-highmem.

yes.

> But the real reason of this work is for huge 64bit archs, so we speedup
> and avoid to waste tons of ram.

pte_chain space consumption is approximately equal to pagetable page space
consumption.  Sometimes a bit more, sometimes a lot less, approximately
equal.

So why do you say it saves "tons of ram"?

> on 32-ways the scalability is hurted
> very badly by rmap, so it has to be removed (Martin can provide the
> numbers I think).

I don't recall that the objrmap patches ever significantly affected CPU
utilisation.

I'm not saying that I'm averse to the patches, but I do suspect that this is
a case of large highmem boxes dragging the rest of the kernel along behind
them, and nothing else.
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