Re: FILEDESC_LOCK() implementation

On Wed, 21 Jun 2006, John-Mark Gurney wrote:

> Kris Kennaway wrote this message on Mon, Jun 12, 2006 at 01:41 -0400:
>> I fixed mutex profiling to a) not be as wrong and b) not suck so very much, 
>> and here is a revised profiling trace from mysql supersmack on a 12 cpu 
>> E4500, sorted by ratio of cnt_lock/count; filedesc lock contention (via 
>> FILEDESC_[UN]LOCK()) is the major mutex contention problem.
>
> Should we also look at breaking down filedesc lock to have multiple locks 
> over the range?  I am thinking of writing a program that will have 32 
> threads (sun4v) and all threads will be doing heavy i/o, and will be even 
> more heavily contested on FILEDESC than the supersmack benchmark would be...
>
> Though this doesn't solve the problem of all 32 threads trying to do i/o on 
> a fd in the same block though...

src/tools/tools/netrate/{http,httpd}, running in threaded mode (-t).  http is 
a client, and accesses lots of independent fds from different threads, 
contending the filedesc lock but not a single fd lock, whereas httpd will do 
both, due to accepting connections.

I would optimize very carefully here, the trade-offs are tricky, and we may 
find that by making locking more complex, we cause cache problems, increase 
lock hold periods, etc, even if we decrease contention.  I've wondered a bit 
about a model where we loan fd's to threads to optimize repeated access to the 
same fd by the same thread, but this mostly makes sense in the context of a 
1:1 model rather than an m:n model.

Robert N M Watson
Computer Laboratory
University of Cambridge