Author: Bruce Moreland
Date: 07:08:11 07/29/98
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On July 29, 1998 at 08:08:01, Komputer Korner wrote: >"Current Win 95 versions load programs by first copying the executable file into >disk cache memory, as shown in "Windows 95, Windows 98 Unaligned." From there, >the OS copies the one or more program modules in the file to another block of >memory, aligning each on a 4-kilobyte page boundary required by the virtual >memory manager. [rest of quote snipped] This is talking about how the executable file is loaded into RAM. This would be done before the first instruction is executed, obviously. I don't know a lot about this, but this sounds like a boot-time issue that wouldn't mean very much, especially on a computer with sufficient RAM. The EXE will probably be under a megabyte, they'd just read it into the disk cache, allocate a new block, and copy it into the new block. *Then* the app gets to run, and allocate its hash table, and do the other nasty things that chess programs do. We're talking about small amounts of memory here, during the earliest phases of application boot. >If the hash table isn't duplicated then why do all the programs except Fritz ( >and even Fritz sneakily disallows more than 50% of RAM for hash tables) swap to >the hard disk whenever a hash table larger than 50% of RAM is used? So either >Windows 95 is grabbing the available RAM for itself or 2 copies of the hash >table are temporarily loaded. This is just wrong. The thing you quoted talked about an inefficiency involved in taking a file off the disk and turning it into something that could be executed. Something is read from disk, diddled, then copied into a new memory block. There is nothing that would point to something more sinister -- that every memory allocation would be done twice, or that anything that is read from the disk involves two memory allocations somehow. The most major deviation from reality in what you say involves the word "loaded". You can talk about an EXE being loaded, but it doesn't make any sense to talk about a hash table being "loaded". A hash table is allocated, not loaded. You request memory from the operating system and it gives it to you. The minor problem you mention, involving the exe being loaded twice, is a higher level issue than this, and it is easily understood by anyone via the quote you included. This other problem you contend exists would involve the memory allocator being fundamentally broken, for no conceivable reason, and in a manner that has no conceivable relation to the other problem. I don't know what design decisions Fritz made regarding their hash table, but let's discuss this for a moment. You have a chunk of memory that will be accessed randomly (meaning at an unpredictable point) several hundred thousand times per second. This is basically a tub of anthrax waiting to kill everyone. When I was in college we had this cheesy HP 1000 minicomputer that ran BASIC and nothing else, and that is how we all learned how to program there. You could do a lot of cool things in that BASIC, but it was pretty hard to destroy the computer. In early 1984 we got a brand new Eclipse MV/10000, with an incredible amount of disk and RAM (a gig of disk and six megs of RAM). This only cost about a half million dollars. It had virtual memory, and we very quickly discovered that we could slag that machine by simply writing something that allocated more than six megabytes of RAM and then accessed it randomly. The machine wouldn't crash, but it would freeze until the OS figured out that someone was being a jerk, at which point it would prevent that application from stealing pages from other applications, which would cause it to sit there in a little cell, eating its own flesh like crazy, and not annoying others very much. But this was annoying at the start, because it took the OS like 30 seconds to figure it out, during which time everyone was looking around going "What! Is the machine crashed?", and when the thing came back it took everyone by surprise and their snakes crashed into the wall and they had to start a new game. Bummer. My point is if the hash table is too big, you get thrashing that is beyond anything that the machine can handle. So the tendency is to not allow the hash table to get too big, and that is probably what Fritz did here. They don't want to get into conflict with the OS and they don't want to get into conflict with other applications, so they play it safe. It is exactly what I would do as well. bruce
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