Re: Mersenne: Two L-L tests at once?
On Thursday 28 February 2002 22:03, Guillermo Ballester Valor wrote: Hi, On Thu 28 Feb 2002 22:19, Brian J Beesley wrote: [ snip ] The difference here is that your method generates memory bus traffic at twice the rate George's method takes advantage of the fact that (with properly aligned operands) fetching the odd element data automatically fetches the adjacent even element data The streams would be alternated : stream0_data(n) , stream1_data(n), stream0_data(n+1), stream1_data(n+1) When fetching data(n) for a stream we also get the other Yes, this scheme does seem to work The memory bottleneck was the first thing I thought, and I was near to discard the idea when I realized that the trig bata would be the same, and the required memory access would be less than double the single stream scheme If a double stream version cost less than double the single one the we can speed up the project a bit On Friday 01 March 2002 00:37, George Woltman wrote: Well, that would be true if SSE2 had a multiply vector by scalar instruction That is, to multiply two values by the same trig value, you must either load two copies the trig value or add instructions to copy the value into both halves of the SSE2 register I can't see that being a major problem Surely there's only one main memory fetch to load the two halves of the SSE2 register with the same value, and surely the loads can be done in parallel since there's no interaction ( M - X; then X - R1 X - R2 in parallel, where X is one of the temporary registers available to the pipeline) On Thursday 28 February 2002 21:20, Steinar H Gunderson wrote: Testing a number in parallel with itself is obviously a bad idea if there occurs an undetected error :-) Sure But the only way there would be a problem here (given that the data values are independent because of the different random offsets) is if there was a major error like miscounting the number of iterations This is relatively easy to test out I'm sort of marginally uneasy, rather than terrified, about running a double-check in parallel with the first test on the same system at the same time Also, I think most people would rather complete one assignment in time T rather than two assignments in time 2T with both results unknown till they both complete Against this is that Guillermo's suggestion does something to counter the relatively low rate at which DCs are completed Regards Brian Beesley _ Unsubscribe list info -- http://wwwndatechcom/mersenne/signuphtm Mersenne Prime FAQ -- http://wwwtasamcom/~lrwiman/FAQ-mers
Re: Mersenne: Two L-L tests at once?
Hi, On Friday 01 Mar 2002 21:22, Brian J Beesley wrote: [ snip ] The memory bottleneck was the first thing I thought, and I was near to discard the idea when I realized that the trig bata would be the same, and the required memory access would be less than double the single stream scheme If a double stream version cost less than double the single one the we can speed up the project a bit On Friday 01 March 2002 00:37, George Woltman wrote: Well, that would be true if SSE2 had a multiply vector by scalar instruction That is, to multiply two values by the same trig value, you must either load two copies the trig value or add instructions to copy the value into both halves of the SSE2 register I can't see that being a major problem Surely there's only one main memory fetch to load the two halves of the SSE2 register with the same value, and surely the loads can be done in parallel since there's no interaction ( M - X; then X - R1 X - R2 in parallel, where X is one of the temporary registers available to the pipeline) We would have to evaluate the cost of memory traffic to load data with two halves the same, or load two differnt data and then double them in two XMM registers I have not any skill in SSE2, no machine to try This morning I've been reading (on the fly) the intel PDF manual, and I saw that the SSE2 was made by Intel engineers thinking more in multimedia than in Mathematics (or GIMPS) There are some elemental ops they could be implemented to do complex number multiplication easy, or a vector by escalar mul, or an exchange within halves :-( Perhaps in SSE3 :) On Thursday 28 February 2002 21:20, Steinar H Gunderson wrote: Testing a number in parallel with itself is obviously a bad idea if there occurs an undetected error :-) Sure But the only way there would be a problem here (given that the data values are independent because of the different random offsets) is if there was a major error like miscounting the number of iterations This is relatively easy to test out I'm sort of marginally uneasy, rather than terrified, about running a double-check in parallel with the first test on the same system at the same time Also, I think most people would rather complete one assignment in time T rather than two assignments in time 2T with both results unknown till they both complete Against this is that Guillermo's suggestion does something to counter the relatively low rate at which DCs are completed I also was worried about that idea, but every time I think, it seems less absurd to me OTOH, I don't know how difficult would be the carry and normalization code of DWT for two _different_ exponents At first approximation, I recall some code I wrote without branches for Glucas, actually a code which makes two streams at once I mean perhaps the cost is small Regards Guillermo _ Unsubscribe list info -- http://wwwndatechcom/mersenne/signuphtm Mersenne Prime FAQ -- http://wwwtasamcom/~lrwiman/FAQ-mers
Re: Mersenne: Two L-L tests at once?
Hi again: I received the mail from Mersenne list two times Is it because of subejct? :) The first time is the mail I sent to list, the second is the same mail mirrored by an unknown for me 'ntsys24yucombe' system !? Back to the subject, I'm wondering about how fast can we do two L-L test in parallel using this SSE2 extensions Basically, I'm thinking in use two nearest exponents with the same FFT-length The memory access in FFT phase would be the same, the trig data also the same, the most difficult part would be the carry-and-normalization pass This dificult could also dissapear making the second test over the same exponent We then get the L-L and double check at once Remember the scheme Prime95 uses to make double check is to shift initally a random number of bits DWT scrambles the data enough to be reasonabily sure both test are independent A matching result would imply a very confident result A non matching result would say us something was wrong It also would allow us to check interim results to be sure all is well so far Regards Guillermo _ Unsubscribe list info -- http://wwwndatechcom/mersenne/signuphtm Mersenne Prime FAQ -- http://wwwtasamcom/~lrwiman/FAQ-mers
Re: Mersenne: Two L-L tests at once?
At 11:03 PM 2/28/2002 +0100, Guillermo Ballester Valor wrote: The memory bottleneck was the first thing I thought, and I was near to discard the idea when I realized that the trig bata would be the same, and the required memory access would be less than double the single stream scheme. Well, that would be true if SSE2 had a multiply vector by scalar instruction. That is, to multiply two values by the same trig value, you must either load two copies the trig value or add instructions to copy the value into both halves of the SSE2 register. Yes, I was thinking in copy the trig value from a half to other, although I don't know how would be the cost. _ Unsubscribe list info -- http://www.ndatech.com/mersenne/signup.htm Mersenne Prime FAQ -- http://www.tasam.com/~lrwiman/FAQ-mers
