Author: J. Wesley Cleveland
Date: 12:58:00 07/22/03
Go up one level in this thread
On July 22, 2003 at 14:27:45, Robert Hyatt wrote: >On July 22, 2003 at 12:28:39, J. Wesley Cleveland wrote: > >>On July 22, 2003 at 08:07:18, Gerd Isenberg wrote: >> >>>On July 21, 2003 at 15:35:17, J. Wesley Cleveland wrote: >>> >>>>On July 18, 2003 at 23:45:16, Robert Hyatt wrote: >>>> >>>>>On July 18, 2003 at 21:58:18, J. Wesley Cleveland wrote: >>>>> >>>>>>On July 18, 2003 at 21:17:14, Robert Hyatt wrote: >>>>>> >>>>>>>On July 18, 2003 at 15:21:35, J. Wesley Cleveland wrote: >>>>>>> >>>>>>>>On July 17, 2003 at 18:25:51, Robert Hyatt wrote: >>>>>>>> >>>>>>>>>On July 17, 2003 at 17:35:33, Dieter Buerssner wrote: >>>>>>>>> >>>>>>>>[snip] >>>>>>>>>> >>>>>>>>>>I cannot find any randomness in the reads of lm-bench (I downloaded latest >>>>>>>>>>stable source today, not the experimental version, available, too). If it would >>>>>>>>>>do random reads, it would have no way to avoid the problem with the TLBs you >>>>>>>>>>explained. >>>>>>>>> >>>>>>>>>4M pages solves it for at least 250mb worth of RAM. But then again, _no_ chess >>>>>>>>>program depends on purely random memory accesses to blow out the TLB. The only >>>>>>>>>truly random accesses I do are the regular hashing and pawn hashing, which >>>>>>>>>both total to significantly less than the total nodes I search. Which means >>>>>>>>>the TLB penalty is not even 1% of my total run time. Probably closer to >>>>>>>>>.01% - .05%. >>>>>>>>> >>>>>>>>>I ignore that. >>>>>>>> >>>>>>>>Why do you think it is that low? I get ~20-30% of nodes have hash probes with >>>>>>>>crafty. >>>>>>> >>>>>>> >>>>>>>Look at the code. >>>>>>I not only looked at the code. I *instrumented it*. I won't have complete >>>>>>results until Monday, but it appears that crafty spends 3-5% of its total time >>>>>>inside hashprobe on my (slow) machine and a prefetch could reduce that by about >>>>>>half. >>>>>> >>>>>>>Crafty probes memory _once_ for a hash probe. That >>>>>>>introduces a memory access penalty once per node in the basic search, >>>>>>>less than once per node in the q-search (I only probe phash there and I >>>>>>>don't probe it but about 25% of the q-search nodes I visit). >>>>>> >>>>>>If you had read whai I wrote, you would see I said crafty does a hash probe >>>>>>20-30% of its total nodes. >>>>> >>>>>OK. I clearly mis-read what you meant. the 20-30% was eye-catching as that >>>>>is a pretty common hash hit percentage as well... >>>>> >>>>> >>>>>> >>>>>>>As a result, you get less than one probe per node searched. A node searched >>>>>>>requires something on the order of 3000-5000 instructions. What percentage >>>>>>>of that 3K-5K instruction timing is that single hash probe? Almost zero. >>>>>> >>>>>>Except that a fast machine may do these 3-5K instructions in <1usec. A cache >>>>>>miss + a TLB miss may take 300-400 ns. I would not call 30% almost 0. >>>>> >>>>>You are missing my point. In the position(s) you tested, you saw 20-30% >>>>>hash probes. That means one probe for every 3-5 nodes. At 1M nodes >>>>>per second, that is 200K-300K probes per second. If you measure the >>>>>time spent in searching a single node, multiply that by 3-5X, then compare >>>>>that to the hash probe time, the time spent probing the hash table is low. >>>>> >>>>>Note that your 5% is _not_ the total time used to probe the table. It is >>>>>the time to probe the table, and do it _twice_ although the second probe >>>>>doesn't have any memory access penalty associated with it in most cases. >>>>> >>>>>So a big percent of that 5% is doing the actual work done in HashProbe(), >>>>>rather than being all memory access penalty... >>>> >>>>I ran some tests on my slow (450 Mhz) machine. Hash was set to 192Mb. The test >>>>was 21 middle-game positions and ran for nearly 1 hour. Crafty got between 125k >>>>and 230k nps. Crafty spent 3.6% of total time in HashProbe. I added the >>>>following code just before the call to RepetitionCheck() in search.c (slightly >>>>modified from the code in hash.c). Note that the code is basically a no-op as >>>>all variables are local. >>>> >>>>{ >>>> static BITBOARD word1; >>>> BITBOARD temp_hashkey; >>>> HASH_ENTRY *htable; >>>>/* >>>> ---------------------------------------------------------- >>>>| | >>>>| first, compute the initial hash address and choose | >>>>| which hash table (based on color) to probe. | >>>>| | >>>> ---------------------------------------------------------- >>>>*/ >>>> >>>> temp_hashkey=(wtm) ? HashKey : ~HashKey; >>>> htable=trans_ref_a+((int) temp_hashkey&hash_maska); >>>> word1=htable->word1; >>>>} >>>> >>>>Now crafty spends 2.8% of its time in HashProbe. >>> >>>Hi Wesley, >>> >>>that's interesting, it seems that preloading decreases the hash-latency. >>>May be prefetching with Athlons's/Opteron's/P4's PREFETCHNTA, (bypassing >>>L2-Cache) is even better. >>> >>>Gerd >> >>I'm sure it would be better. My code doesn't make it run any faster, it just >>shows that the delay due to memory access is significant. >> > >Can you tell me how you conclude this? The *only* effect of the code I added is to ensure that the depth-preferred part of the hash table is put into cache, so any speedup in HashProbe is due to not having a cache (and ATB) miss. > >IE there are two parts in HashProbe(); > >1. probe "depth-preferred table". > >2. probe "always-store" table". > >You are assuming that of the total 3.6% done in HashProbe(), that .8% is >done in the always-store code. Which means that .8% is done in the depth- >preferred table, and the remaining time is memory latency. > >I don't think that is the explanation. > >Suppose _many_ hits occur in the depth-preferred table. Then you won't be >probing the always-store table at those positions. And your .8% assumption >is not so safe to make. Unless you run huge searches with a small table, >this effect will distort any possible conclusions. > > >No way a single random access memory read is 3% of the total time spent >doing a node. There are way too many _other_ random-access reads done in >crafty to make that possible. The total time would go over 100%. At 1M nodes/sec, the time for 1 node is (obviously) 1 usec. The latency for one cache miss is about 150 nsec. This implies that if you have *one* cache miss every 4 nodes, you will spend 3% on that single random access memory read. Apparently, caching works very well for crafty, except for HashProbe( ;). Again, my figures are on my slow machine. Your machine is ~6x faster, while your memory latency is not much better, so I suspect the figures will be much worse on your machine. You may want to test this for yourself. > >>> >>> >>>{ >>> static BITBOARD word1; >>> BITBOARD temp_hashkey; >>> HASH_ENTRY *htable; >>>/* >>> ---------------------------------------------------------- >>>| | >>>| first, compute the initial hash address and choose | >>>| which hash table (based on color) to probe. | >>>| | >>> ---------------------------------------------------------- >>>*/ >>> >>> temp_hashkey=(wtm) ? HashKey : ~HashKey; >>> htable=trans_ref_a+((int) temp_hashkey&hash_maska); >>>#ifdef _DOPREFETCH >>> __asm mov eax, [htable]; // get the pointer >>> __asm PREFETCHNTA [eax]; // fetch to L1-cache, bypassing L2-Cache >>>#else >>> word1=htable->word1; >>>#endif >>>} >>> >>>some additional notes from: >>> >>>"AMD Athlon™ Processor x86 Code Optimization Guide" >>> >>>Prefetching versus Preloading >>> >>>In code that uses the block prefetch technique as described in >>>“Optimizing Main Memory Performance for Large Arrays” on page 66, a standard >>>load instruction is the best way to prefetch data. But in other situations, load >>>instructions may be able to mimic the functionality of prefetch instructions, >>>but they do not offer the same performance advantage.Prefetch instructions only >>>update the cache line in the L1/L2 cache and do not update an architectural >>>register. This uses one less register compared to a load instruction. Prefetch >>>instructions also do not cause >>>normal instruction retirement to stall. Another benefit of prefetching versus >>>preloading is that the prefetching instructions can retire even if the load data >>>has not arrived yet. A regular load used for preloading will stall the machine >>>if it gets to the bottom of the fixed-issue reorder buffer (part of the >>>Instruction Control Unit) and the load data has not arrived yet. The load is >>>"blocking" whereas the prefetch is "non-blocking."
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