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DragonFly kernel List (threaded) for 2005-02
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Re: phkmalloc vs. mmap direct

From: Matthew Dillon <dillon@xxxxxxxxxxxxxxxxxxxx>
Date: Sun, 20 Feb 2005 13:20:02 -0800 (PST)

    OK... if it's simply a hysteresis issue with the current table allocation
    code, then someone go look at the code and fix it so the hysteresis is
    more reasonable.  That would solve at least part of the problem and
    probably not require any major code changes.

					Matthew Dillon 

:These plots are meant to show a particular feature of
:phkmalloc--high latency when fulfilling requests that it
:cannot fit within the existing keg.
:It uses an internal dynamic allocator which it builds around
:the mmap system call.  It is uses this allocator to store a
:construction that it calls the page_dir.  The page_dir is
:something akin to a userspace version of the page table.
:Rather than simply denote a 16MB? address region for this
:table, the table is continually remade.  It is kept to
:fit precisely the number of entries that correspond to 
:the exisiting allocation.  Thus, when you require more
:address space the following sequence of events happens:
:1) brk is called
:2) it is determined the addresses now made available via brk
:   do not fit in the existing page_dir structure.
:3) a new empty page_dir structure is made by using mmap to
:   get a new region
:4) memcpy is used to transfer the old page_dir structure into 
:   the new mmap allocation
:5) munmap is applied to the old region
:This is done in the belief that this conserves the RES size
:of the process--a point which I'm inclined to dispute, as
:phkmalloc is otherwise structured so that you will never
:read or write to an address of memory comprising the page_dir
:unless there is an entry there that corresponds to memory
:that is going to be or was just in use.
:Back when phkmalloc was written, ('96), the situation was
:entirely different.  A program was morely likely to be
:bogged down by paging delays, thus masking this particular
:behavior if the program did anything with the memory it
:Do real programs incur these penalties?  For sure, any
:program that engages in a pseudo lifo allocation
:pattern--i.e., any program that allocates lots of memory
:during a recursive algorithm.

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