linux-kernel - Re: [RFC PATCH 0/5] Add accountings for Page Cache

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Date:	Thu,  3 Mar 2011 10:50:52 +0900 (JST)
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	noname noname <namei.unix@...il.com>
Cc:	kosaki.motohiro@...fujitsu.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, jaxboe@...ionio.com, akpm@...ux-foundation.org,
	fengguang.wu@...el.com
Subject: Re: [RFC PATCH 0/5] Add accountings for Page Cache

> [Summery]
> 
> In order to evaluate page cache efficiency, system admins are happy to
> know whether a block of data is cached for subsequent use, or whether
> the page is read-in but seldom used. This patch extends an effort to
> provide such kind of information. We adds three counters, which are
> exported to the user space, for the Page Cache that is almost
> transparent to the applications. This would benifit some heavy page
> cache users that might try to tune the performance in hybrid storage
> situation.

I think you need to explain exact and concrete use-case. Typically, 
cache-hit ratio doesn't help administrator at all. because merely backup
operation (eg. cp, dd, et al) makes prenty cache-miss. But it is no sign
of memory shortage. Usually, vmscan stastics may help memroy utilization
obzavation.

Plus, as ingo said, you have to consider to use trancepoint framework
at first. Because, it is zero cost if an admin don't enable such tracepoint.

At last, I don't think disk_stats have to have page cache stastics. It seems
slightly layer violation.

Thanks.

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