Re: Syslets, Threadlets, generic AIO support, v6

[Eric Dumazet <dada1@cosmosbay.com>]

On Thu, 31 May 2007 11:02:52 +0200
Ingo Molnar <mingo@elte.hu> wrote:

> 
> * Ingo Molnar <mingo@elte.hu> wrote:
> 
> > it's both a flexibility and a speedup thing as well:
> > 
> > flexibility: for libraries to be able to open files and keep them open 
> > comes up regularly. For example currently glibc is quite wasteful in a 
> > number of common networking related functions (Ulrich, please correct 
> > me if i'm wrong), which could be optimized if glibc could just keep a 
> > netlink channel fd open and could poll() it for changes and cache the 
> > results if there are no changes (or something like that).
> > 
> > speedup: i suggested O_ANY 6 years ago as a speedup to Apache - 
> > non-linear fds are cheaper to allocate/map:
> > 
> >   http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg23820.html
> > 
> > (i definitely remember having written code for that too, but i cannot 
> > find that in the archives. hm.) In theory we could avoid _all_ 
> > fd-bitmap overhead as well and use a per-process list/pool of struct 
> > file buffers plus a maximum-fd field as the 'non-linear fd allocator' 
> > (at the price of only deallocating them at process exit time).
> 
> to measure this i've written fd-scale-bench.c:
> 
>    http://redhat.com/~mingo/fd-scale-patches/fd-scale-bench.c
> 
> which tests the (cache-hot or cache-cold) cost of open()-ing of two fds 
> while there are N other fds already open: one is from the 'middle' of 
> the range, one is from the end of it.
> 
> Lets check our current 'extreme high end' performance with 1 million 
> fds. (which is not realistic right now but there certainly are systems 
> with over a hundred thousand open fds). Results from a fast CPU with 2MB 
> of cache:
> 
>  cache-hot:
> 
>  # ./fd-scale-bench 1000000 0
>  checking the cache-hot performance of open()-ing 1000000 fds.
>  num_fds: 1, best cost: 1.40 us, worst cost: 2.00 us
>  num_fds: 2, best cost: 1.40 us, worst cost: 1.40 us
>  num_fds: 3, best cost: 1.40 us, worst cost: 2.00 us
>  num_fds: 4, best cost: 1.40 us, worst cost: 1.40 us
>  ...
>  num_fds: 77117, best cost: 1.60 us, worst cost: 2.00 us
>  num_fds: 96397, best cost: 2.00 us, worst cost: 2.20 us
>  num_fds: 120497, best cost: 2.20 us, worst cost: 2.40 us
>  num_fds: 150622, best cost: 2.20 us, worst cost: 3.00 us
>  num_fds: 188278, best cost: 2.60 us, worst cost: 3.00 us
>  num_fds: 235348, best cost: 2.80 us, worst cost: 3.80 us
>  num_fds: 294186, best cost: 3.40 us, worst cost: 4.20 us
>  num_fds: 367733, best cost: 4.00 us, worst cost: 5.00 us
>  num_fds: 459667, best cost: 4.60 us, worst cost: 6.00 us
>  num_fds: 574584, best cost: 5.60 us, worst cost: 8.20 us
>  num_fds: 718231, best cost: 6.40 us, worst cost: 10.00 us
>  num_fds: 897789, best cost: 7.60 us, worst cost: 11.80 us
>  num_fds: 1000000, best cost: 8.20 us, worst cost: 9.60 us
> 
>  cache-cold:
> 
>  # ./fd-scale-bench 1000000 1
>  checking the performance of open()-ing 1000000 fds.
>  num_fds: 1, best cost: 4.60 us, worst cost: 7.00 us
>  num_fds: 2, best cost: 5.00 us, worst cost: 6.60 us
>  ...
>  num_fds: 77117, best cost: 5.60 us, worst cost: 7.40 us
>  num_fds: 96397, best cost: 5.60 us, worst cost: 7.40 us
>  num_fds: 120497, best cost: 6.20 us, worst cost: 6.80 us
>  num_fds: 150622, best cost: 6.40 us, worst cost: 7.60 us
>  num_fds: 188278, best cost: 6.80 us, worst cost: 9.20 us
>  num_fds: 235348, best cost: 7.20 us, worst cost: 8.80 us
>  num_fds: 294186, best cost: 8.00 us, worst cost: 9.40 us
>  num_fds: 367733, best cost: 8.80 us, worst cost: 11.60 us
>  num_fds: 459667, best cost: 9.20 us, worst cost: 12.20 us
>  num_fds: 574584, best cost: 10.00 us, worst cost: 12.40 us
>  num_fds: 718231, best cost: 11.00 us, worst cost: 13.40 us
>  num_fds: 897789, best cost: 12.80 us, worst cost: 15.80 us
>  num_fds: 1000000, best cost: 13.60 us, worst cost: 15.40 us
> 
> we are pretty good at the moment: the open() cost starts to increase at 
> around 100K open fds, both in the cache-cold and cache-hot case. (that 
> roughly corresponds to the fd bitmap falling out of the 32K L1 cache) At 
> 1 million fds our fd bitmap has a size of 128K when there are 1 million 
> fds open in a single process.
> 
> so while it's certainly not 'urgent' to improve this, private fds are an 
> easier target for optimizations in this area, because they dont have the 
> continuity requirement anymore, so the fd bitmap is not a 'forced' 
> property of them.

Your numbers do not match mines (mines were more than two years old so I redid a test before replying)

I tried your bench and found two problems :
- You scan half of the bitmap
- You incorrectlty divide best_delta and worst_delta by LOOPS (5)

Try to close not a 'middle fd', but a really low one (10 for example), and latencie is doubled.

with a corrected bench; cache-cold numbers are > 100 us on this Intel Pentium-M

num_fds: 1000000, best cost: 120.00 us, worst cost: 131.00 us

On an Opteron x86_64 machine, results are better :)

num_fds: 1000000, best cost: 28.00 us, worst cost: 106.00 us
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