Author: Vincent Diepeveen
Date: 12:29:30 07/12/03
Go up one level in this thread
On July 11, 2003 at 13:02:12, Robert Hyatt wrote: >On July 11, 2003 at 06:26:12, Vincent Diepeveen wrote: > >>On July 10, 2003 at 16:50:45, Robert Hyatt wrote: >> >>Pingpong does *not* need a processor to get wakened up. Don't know which OS you >>use that might be doing that, but the ping pong for MPI when properly >>implemented does *not* wake up processes at all. > >Vincent that just shows how _little_ you understand about what is going >on. When you send a packet to a remote machine, _something_ has to read >that packet. That _something_ is a process, and it is blocked until the >packet arrives. > >That's why my ping-pong test is done differently. > > >> >>I hope you will understand that. If pingpong would wait for a process to wake up >>it would run at 10 ms because a process can wake up at most 100 times a second >>in linux (and all *nix flavours) as the scheduler runs 100Hz. > >That is _incorrect_. > >Unix (Linux in particular) will only context switch every 10ms or so, to Being someone with a bad memory, you really should more accurately write down the stuff you already had guessed wrong in the past. A month or 6 ago you guessed it wrong here in CCC. Now you guess it wrong again. To clear your memory, a month or 6 ago i very upset posted that letting my processes idle using spin_lock performed very bad. I had forwarded my questions on this to the IRIX OS guys. Note that they use linux too at the new systems, so this was a very on topic question at the time to post about. In fact this is relevant to redhat linux 7.2 too. If not directly unlocked (within 600 ns or so) within the kernel, getting a new lock from the system means that a locked process when achieving the lock gets put back in the run_queue. The run_queue executes at 100 Hz in all *nix systems. The reason for that, even though they could make it way faster nowadays, is that some very important software is assuming it is running at 100Hz. Therefore the latency is at least 10 ms. When i upset posted that here, i got as answer that windows was not a hair better as it had a typical latency of 15 ms to wake up, i did not get that from the NT kernel team, but from someone posting here. I am getting very sick of you getting back to the same wrong lemma's each time. This was posted very clearly. Trivially a searching thread of crafty/diep will be running already at the processor. It is clear that the MPI is not needing wake up time of a process to work. If it would then the researchers would be mad to use MPI. It is *not* doing that however. Getting put in the run_queue each time is no fun :) >control context switching overhead. But if _nothing_ is ready to run, and >a process unblocks, it does _not_ wait another 10ms before running. It >runs _right now_. > > >> >>If you do not care for the pingpong test then you obviously do not care for >>chessprograms as well. Because if you need a hashtable entry you definitely need >>that latency test to measure. > >No, I just run the ping pong test _correctly_ to see what the real latency >of the hardware is. Software/scheduling latency is _another_ issue that may >or may not affect a chess program. IE I don't do blocking I/O so I don't >have to get "woken up" ever. > >> >>Note that all HPC professors do include pingpong in the first tests they use to >>measure supercomputers/clusters. >> >>In fact out of the x cluster/supercomputer dudes i asked after pingpong test i >>got within a second answer out of all of them. For them the pingpong test *is* >>very relevant. > >It is relevant. How it is done is _also_ relevant. > >If you measure latency as you are doing, you will get different answers on >different systems. Windows and Linux are different and you are measuring >both software _and_ hardware latency with your method. Same problem if you >compare windows to solaris, or IRIX, or any other unix variant (or non-unix >if you want.) > >My latency measure answers the following question: > >"How long does it take me to send a packet to the remote machine and have >it receive it?" Independent of the Operating System. Independent of the >application. Just "what can the hardware do best-case?" > >I know _that_ number precisely. And if I want to write a chess program that >uses that hardware, and I want _that_ latency, I can certainly get it by doing >my own direct hardware interface. If I want to be more portable, and absorb >some O/S latency on top of the hardware latency, I'll do it a different way. > >However, I _have_ been using the term "hardware latency" and I have most >certainly measured it very precisely. And if you'd like to see me bounce >a packet back and forth in just over a usec, I'll be happy to do so. > >Whether you can do it or not is irrelevant, of course. > >Because I didn't say _you_ could do it. I said _I_ had _done_ it.
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