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Re: 2nd attemt at reviving the filesystem limit discussion.

From: Neal H. Walfield
Subject: Re: 2nd attemt at reviving the filesystem limit discussion.
Date: 06 Dec 2002 16:11:03 -0500
User-agent: Gnus/5.0808 (Gnus v5.8.8) Emacs/21.2

> I wonder what ideas Neal had, I couldn't find a thread where he described his 
> ideas.

My idea is to maintain a ~1GB area of "metadata control space."  This
area, rather than being a one-to-one mapping of memory to backing
store (as it currently is), would lazily map (via a hash) the metadata
lazily as it is requested.  (The hash would be used to save the active
mappings.)  When a page of meta-data is requested, the hash table is
consulted, if the page is already mapped, the virtual address is
returned; if not, free space in the 1GB area is allocated.  The
pager_read_page and pager_write_page routines would consult these
hashes and read the data from or write the data to the right area of
the disk as appropriate.

My method differs from Thomas' in that I only worry about meta-data.
Additionally, the mappings are torn down when the Mach thinks it is
appropriate; not immediately after the region is used or by some other
internal algorithm.

The advantage to this algorithm over Roland's is that there would be
fewer system calls--no need to create and rip down memory objects.
Also, the startup time would be cheaper as the meta-data is allocated
lazily rather than a Grand Reorganization when the translator is

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