"Andy Walker" wrote in message
...
In article , Guest
wrote:
(a) Even in a well-written engine, you may want/need to "clear
out" the table from time to time. Initialising several terabytes
may take a little while even on a modern machine ....
Actually, current programs don't do that. They leave the hash alone.
They
don't even initialize it when the program is first run.

I don't have access to the source code of the commercial
programs, so you seem to have an advantage in this respect. But:

I don't really either. But the methods have been widely discussed for
years.

(Well, I do have the same kind of access that everybody else has. The early
Fruit & Toga, Strelka (the reverse enginered Rybka), Crafty, and so on.
However I generally make a point not to look at them. I much prefer people
talking about chess programming ideas and what they do rather than actually
looking at the code itself.)

Sure, you can clear out the memory. That'll work, and for most memory sizes
it wont take an excessive amount of time.

Or you can leave it in and use the results for the next search. That's been
done by programs too.

The current methods of invalidating at time of access have really only
become used in the last few years, when processors got so much faster than
memories.

In the older days, probably right up to the P3 days, processor speeds were
closer to main memory, and memory sizes were small enough that directly
clearing it wasn't a problem.

I don't think even the mainframe programs worried about it. (Although I
can't say for sure, since there weren't too many mainframe chess programs
left after the mid-80s.)

Right off hand, I don't know when the 'invalidate at access' ideas actually
came about.



Either they actually use the data from the previous search, or they
invalidate it at the time of access...
There are several ways to invalidate at access... [...]

OK. But they all involve trade-offs, inc extra computation
and/or extra data stored in the table; which way the trade-offs

Just an extra 64 bit XOR. Just a couple cycles per hash check. And
considering you'll probably be spending 200 to 400 cycles waiting for a
random access to main memory, it's neglible.

Some people prefer to use a few extra bits in the hash entry to hold a
search ID number. It depends on how much info you try to fit into a hash
entry and what cache line size you expect. Since you don't want a hash
entry (or group of entries) to span more than one cache line, you may have a
few extra bits to spare for a unique search ID.

balance would seem to depend on depth of search, expected number
of reads vs writes, relative speed of memory, etc., and it would
be at least a slight surprise if they balanced the same way for
all engines on all hardware for all time.

That is true.

There are lots of ways to do things in a chess program.

And as I said above, it's only been relatively recent that cpu's became so
much faster than main memory, so in the 'old days' it just didn't matter.


(b) Some of the standard operations on a hash table involve
indexing and finding remainders modulo the table size. [...]

It is true that doing an AND is faster than a modulo. But the time
difference isn't really all that significant considering how long main
memory is going to take to access. [...]

OK, but DR's contention was that swapping was the *only*
reason not to have the table as large as possible. Presumably,

It is the main reason. But not the only one.

it is very likely that a table with [eg] 2^25 entries is better
than one with 2^25 + 1?

Not really any great reason for it to be.

There would only be a few reasons it might.

1) The extra size of the hash table.

Not too likely one more entry is going to matter! Even doubling a large
cache is only going to help a few ratings points once you get past a
reasonable size. (I don't have any current numbers for that.)


2) The odd size of the hash table might spread out the entries more or less
uniformly.

Some truth to that, but for chess hashes, which use random Zobrist keys,
it's not really a major issue. Probably wouldn't hurt, though.


3) It being easier to make a hash index out of the 64 bit hash value.

Working with powers of two (or +1 or -1 of 2^x) is certainly more
convenient. But not really a major problem.

True, a 64 bit divide is likely to take around 64 cycles. Some cpu's will
be faster and some slower. But slow. Some may even be "dead dog" slow.

But you (or the compiler) can probably schedule things so you can do some
other stuff while waiting for the divide to finish.

If you are on a system where you know it has a slow divide and you still
want an odd size hash, all you have to do is multiply by the reciprocal.
Since you'll be working with one size hash for the whole program run, it'll
be quick & easy to set up.

And since we are doing this for an index rather than a math answer, you wont
even need to do a 'fix-up' to get the exact right answer. It being off by
one will be tolerable since it will be consistantly off by one for that
index. (But if you want to do the fix-up, it'd take only another few
cycles. Still much faster than a divide.)

Do any current programs do it that way.... (shrug) I have no idea. I
haven't heard anybody actually say.

But it's a simple tweak so I would certainly assume at least some do if they
want to use odd size hashes. If you have 4g of memory, the OS and the
program is going to use some, so you'd be left with about 1.5g of unused
memory if you used only powers of two..

Or they may just go ahead and do a power of two but do a multi-bucket hash
entry. They may be able to fit 3 entries per cache line, so their cache
size would actually be 3*2^x.


There's a lot of room for doing things different ways. It depends a lot on
personal preference and what you know about your target hardware.




And as for the 'ints' vs. 'longs'... Sorry. That was true in the days of
16
bit compilers, but these days on 32 bit systems, both int & long will be
32
bits. And on a 32 bit system, 32 bits will be enough to access all of
physical memory. [...]

I was recently using a system with 32-bit ints and 64-bit
longs and pointers; I didn't think it particularly unusual. But
perhaps it was.

I'm not sure which you are meaning.

1) An unusual system where the cpu really does work with 32 bit ints but
extended pointers.

Well... there are quite a few non-x86 systems around. And not all of them
do things the same way.

Admittedly I am more familiar with regular x86 style consumer processors,
rather than RISC, mainframes, workstations, etc.

And the C89 & C99 standards provide quite a bit of flexibility in how big
things can be. You can indeed have a 32 bit system with a 64 bit address
space. The C standard doesn't really care what the system requires or even
what the compiler writer chooses to do (61 bit integers and 47 bit pointers
on an 8 bit micro, for example.)


2) Simply an extended 32 bit cpu, where the pointers have been hacked in the
cpu to be longer. Somewhat like the segment registers of the old 8088 or
the stuff Intel put into its servers before they were ready to release 64
bit cpus.

I haven't used those Intel servers, but I don't think you can access the
extra memory directly. You have to go through some hoops to acess the extra
memory.

Would the cost be excessive for that increased hash size... Don't know.
That'll depend on the chess program & the cpu.

However, the extra cost for each access is not going to be large. It'd
probably be swamped by the slow memory access times. If main memory often
takes 300 cycles for a random access, then what is an extra 4-10 cycles?
Your program is still at a dead stop waiting for the hash entry to be
returned.


3) A real 64 bit cpu where the compiler just happens to use 32 bits for an
int but 64 for longs and pointers.

In this case, it doesn't matter. It's still all 64 bit hardware. It's not
like a 32 bit cpu working with 64 bit math.

The only issues you have to deal with are the unspecified & inconsistant
sizes of int & long. The C standard doesn't really care as long as
sizeof(short) = sizeof(int) = sizeof(long). You could have all of them be
64 bits, if you wanted. It's so hopelessly screwed up that the C99 standard
gave up and created brand new data types that you should use whenever size
is important. I wouldn't be surprised if the next C standard actually
started depricating the use of 'int'.

So it is certainly possible you have a 64 bit system where the C compiler
does 32 bit ints and 64 bit longs. That's just the compiler. The hardware
is still in 64 bit mode, so it's not an issue.

You will likely have issues if you switch compilers, though. That's why
it's better to use the new data types that C99 specified. I generally never
use 'int' for anything except general vars, loop indexes, etc. where there
is never any possibility that it would even aproach 32 bits. (Yeah, I am
kind of gambling that my programs will never be ported to a system that has
less than 32 bit integers. Which is probably safe....)



But I guess I shouldn't have done a blanket statement like that. (grin) I
guess I sit corrected....


Now, having said all of that....


Hashes are going to be at least 64 bits. Nothing smaller is going to be
acceptable. 32 bits stopped being tolerable back in the 70s or early 80s, I
guess. 48 bits by the early 80s. At 64 bits, the false hash hit rate on
current processors is low enough that it's not a major concern (provided the
occasional false hash hit doesn't crash your engine. That's why some
authors always verify the suggested move from the hash.)

So you aren't really going to be getting into 'int' vs. 'long' anyway.

About the only issue would be whether or not your cpu has 64 bit modulo or
not. If it doesn't, then a power of two might be a better choice else you
might have to do a slower 'software' based modulo.

You can do a full cache line of hash buckets per entry. That way you get
the advantage of using more memory than a simple power of two while at the
same time keeping the hash-index conversion quick & simple.

If you have terabytes of memory, then you probably have a 64 bit cpu, with a
full 64 bit ALU with a full 64 bit divide (and mul) instruction.



(c) If you manage to lock down all, or most, of the storage of
a machine, you may perhaps be unpopular, in a multi-user environment,
Well DUH.
However, on a multi-user environment you are likely to have memory
limitations imposed so you can't use more than your fair share. In fact,
you may only be given a couple meg of physical memory and all the rest of
your data will be paged in & out as needed. Definetly not a good thing
for
large hash tables.

I don't think I've used a system like *that* since the days when
2M of physical memory was more than "elephantine"! ...

Well, a couple meg of memory was just an example.

But no multi-user environment today is going to let one user hog all the
resources of the system. I don't think even the most primative multi-user
systems in use today lets a user get away with that. They wouldn't be much
of an OS if they did.

If you are on a 1g system with 8 other people, no reasonable multi-user OS
is going to let one user have 900meg of that and starve the rest of the
users. Not without elevating their privileges.

The amount of physical memory each user gets is likely to be somewhat
variable, changing depending on the load etc. But the user is likely to
only be 'guarnateed' a small amount of memory. Everything else will be
paged to disk as needed and you can't really count on any particular amount
of memory for a hash.

So it wouldn't be a good idea for a user to even try to do a big hash. It
just wouldn't work well, the program would run slowly and it'd play terrible
because stuff would be paging to & from disk.

[...]
And if you are on a system without that restriction and you don't have
agreements etc. and you do it anyway and cause problems... (shrug) Tough.
That's your problem when the sysadmin catches you.

... And ever since those days I've *been* the sysadmin, at
least for the machine on my desk. But not really the point:


If you are runing a chess program for a reason (such as a tournament),
then
you know not to use it for other things.

OK.

If you intend to use it for other things, then just don't tell it to use
all
your available memory for the hash. (Although your other programs will
reduce the cpu power available to the chess program, which may cost it a
full ply of search.)

Then we're back to contention with DR's claim that the larger
the better until swapping occurs. Personally, I've just acquired my
first "Vista" machine [no flowers, by request], with its annoying

I got my first Vista system (a laptop) a few months ago.... Can't say I
like it.

habits of firing off all sorts of stuff to and from M$ without any
notice, suddenly deciding to re-arrange its discs, telling me at

You can turn off the background defrag. Along with much of the other
background & scheduled stuff.

awkward moments to insert such-and-such disc to do dumping, and so

Never had Vista ask me for a disk.

on. But even my Unix box is liable to receive e-mail, or to start
"cron" jobs. None of these things seem likely to occupy that much
CPU, but they do load up some surprisingly large programs.

Right.

On a highly variable system like that.... well, you are screwed, basically.
That's just the nature of having a system that isn't suitable for running a
processor intensive, memory hungry program.

About the best you can do is only use a few meg and hope that'll be enough
and that the OS wont page out even that small amount. Or you live with the
variability.


But on a system that is fairly stable. With no sudden background task etc.
(like my XP box, where very little happens without me doing it), you can get
pretty consistant memory & processor
availability.


And if you do want to run some other program and a chess program with
massive hash tables at the same time.... (shrug) Get a second computer.
It's not like they are expensive. A second computer can be bought for
$400
that's more than good enough for basic usage, web surfing, etc. For just
a
few dollars more, you could get a low end laptop instead.

Perhaps that's a left- vs right-pondian viewpoint. The machines
you can get for that sort of price over here really aren't "good enough"

I don't know where you are. I'm in the U.S. A low end system with a 1.8ghz
AMD x2, 1g and 250g drive would be around $400. A little shopping around
would probably improve that somewhat. Not a massive cost. Eating out at a
fast food resturant 15 times. Eating at a regular resturant 10 times.
10-15 tanks of gas for the car.

Most people (and even families) can blow that much money in a month or two
and never even notice it.

The US, Canada, & UK should have roughly comparable average household
incomes.


at all. If they were merely last year's low-end models reduced to clear,
that might be different, but we only seem to get a teensy discount for
that. Instead, the very cheap ones are totally inadequate. YMMV.

Depends on what you are wanting to do.

The 'average' person is going to surf the web, check email, do some instant
chatting, watch a few youtube videos, etc.

A more advanced person might do a little programming or video or photo
editing, or a game or two.

No serious number crunching or heavy 3d game playing. (Yes, they may like
heavy 3d games, but they probably aren't going to be playing one in the
short time the main system is occupied.)

For that, most low end systems *are* good enough to be a secondary computer.
That's why they are a secondary system, rather than a second main system.

Most low end systems have more computing power than many of the more
expensive systems of just a couple years ago. We just expect more from them
and the OS (Vista) takes up more of the power.


I have two computers at my desk. My laptop and a desktop. Before I got the
laptop, I had two desktop systems hooked up to a KVM switch. My main system
and a slower old secondary system. The second one usually didn't get used
unless I was doing something on the main one that I didn't want to
interrupt, or I was wanting to do a long term program that would run several
days and I didn't want to tie up the main system.


(As a side note, I've read that a lot of chess programmers actually use a
small collection of old or cheap computers to do their chess program tuning
test games. They can get a couple cheap low end computers and do more test
games that way than if they spent the same amount on a high end system.)


As it happens, Prof Kraft [the OP] really gave us no idea what

I have no idea who the author was. I guess you recognised him, though.

hardware or software he was asking about, if any [eg, perhaps he is

Since he was only talking about using up to 1.5g of hash memory, it
obviously was either a small desktop system or a laptop.

A high end desktop would have had more memory. Perhaps up to 4g. A server
would have had more. A mainframe or workstation would have had more.

So it probably was just a plain desktop or laptop.

writing his own program, or perhaps he needs background info for an
academic paper] so all bets are off. Your initial advice ["Basically,

If he's needing background info for a paper, then he's in the wrong
newsgroup! I don't think he wants to hear much about Sam Sloan etc. etc...

He should head over to TalkChess.com That's where the chess programmers
hang out these days.


run a simple test..."] to him is sound, of course. My only concern
was that one should not be quite so confident that "bigger is better"
in all [non-swapping] cases.

Maybe a blanket statement is a little unwise. Just because of the few
situations and systems and such where issues might arise.

But it's not too far off though. Close enough that you could safely say
something like:

"Using as much memory as what's available without paging is nearly always a
good idea."

Just the addition of the word 'nearly' covers the few cases where it
wouldn't.




--
Andy Walker
Nottingham