On Thu, May 20 2010, Linus Torvalds wrote: > > > On Thu, 20 May 2010, Miklos Szeredi wrote: > > > > With Jens' pipe growing patch and additional fuse patches it was > > possible to achieve a 20GBytes/s write throghput on my laptop in a > > "null" filesystem (no page cache, data goes to /dev/null). > > Btw, I don't think that is a very interesting benchmark. > > The reason I say that is that many man years ago I played with doing > zero-copy pipe read/write system calls (no splice, just automatic "follow > the page tables, mark things read-only etc" things). It was considered > sexy to do things like that during the mid-90's - there were all the crazy > ukernel people with Mach etc doing magic things with moving pages around. > > It got me a couple of gigabytes per second back then (when memcpy() speeds > were in the tens of megabytes) on benchmarks like lmbench that just wrote > the same buffer over and over again without ever touching the data. > > It was totally worthless on _any_ real load. In fact, it made things > worse. I never found a single case where it helped. > > So please don't ever benchmark things that don't make sense, and then use > the numbers as any kind of reason to do anything. It's worse than > worthless. It actually adds negative value to show "look ma, no hands" for > things that nobody does. It makes people think it's a good idea, and > optimizes the wrong thing entirely. > > Are there actual real loads that get improved? I don't care if it means > that the improvement goes from three orders of magnitude to just a couple > of percent. The "couple of percent on actual loads" is a lot more > important than "many orders of magnitude on a made-up benchmark". I agree on the basis that these types of benchmarks are fine to validate the "are there stupid problems in the new code?" question, but not so much as a comparison for anything. I can easily run some pure IO benchmarks and send some numbers comparing 64KB vs 1MB pipes on splice. It's been a while since I did that, if I recall correctly then the biggest issue I ran into back then was beating on the inode mutex a lot and not so much the actual syscall entry/exit count being a multiple of comparison tests with read/write using a larger buffer. -- Jens Axboe -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>