Chuck Lever wrote: > On Jan 13, 2009, at Jan 13, 2009, 5:26 AM, Greg Banks wrote: >> Stop gathering the data that feeds the 'th' line in /proc/net/rpc/nfsd >> because the questionable data provided is not worth the scalability >> impact of calculating it. Instead, always report zeroes. The current >> approach suffers from three major issues: >> >> 1. update_thread_usage() increments buckets by call service >> time or call arrival time...in jiffies. On lightly loaded >> machines, call service times are usually < 1 jiffy; on >> heavily loaded machines call arrival times will be << 1 jiffy. >> So a large portion of the updates to the buckets are rounded >> down to zero, and the histogram is undercounting. > > Use ktime_get_real() instead. This is what the network layer uses. IIRC that wasn't available when I wrote the patch (2.6.5 kernel in SLES9 in late 2005). I haven't looked at it again since. Later, I looked at gathering better statistics on thread usage, and I investigated real time clock (more precisely, monotonic clock) implementations in Linux and came to the sad conclusion that there was no API I could call that would be both accurate and efficient on two or more platforms, so I gave up. The HPET hardware timer looked promising for a while, but it turned out that a 32b kernel used a global spinlock to access the 64b HPET registers, which created the same scalability problem I was trying to fix. Things may have improved since then. If we had such a clock though, the solution is very simple. Each nfsd maintains two new 64b counters of nanoseconds spent in each of two states "busy" and "idle", where "idle" is asleep waiting for a call and "busy" is everything else. These are maintained in svc_recv(). Interfaces are provided for userspace to read an aggregation of these counters, per-pool and globally. Userspace rate-converts the counters; the rate of increase of the two counters tells you both how many threads there are and how much actual demand on thread time there is. This is how I did it in Irix (SGI Origin machines had a global distributed monotonic clock in hardware). > You could even steal the start timestamp from the first skbuff in an > incoming RPC request. This would help if we had skbuffs: NFS/RDMA doesn't. > > This problem is made worse on "server" configurations and in virtual > guests which may still use HZ=100, though with tickless HZ this is a > less frequently seen configuration. Indeed. > >> 2. As seen previously on the nfs mailing list, the format in which >> the histogram is presented is cryptic, difficult to explain, >> and difficult to use. > > A user space script similar to mountstats that interprets these > metrics might help here. The formatting in the pseudofile isn't the entire problem. The problem is translating the "thread usage histogram" information there into an answer to the actual question the sysadmin wants, which is "should I configure more nfsds?" > >> 3. Updating the histogram requires taking a global spinlock and >> dirtying the global variables nfsd_last_call, nfsd_busy, and >> nfsdstats *twice* on every RPC call, which is a significant >> scaling limitation. > > You might fix this by making the global variables into per-CPU > variables, then totaling the per-CPU variables only at presentation > time (ie when someone cats /proc/net/rpc/nfsd). That would make the > collection logic lockless. This is how I fixed some of the other server stats in later patches. IIRC that approach doesn't work for the thread usage histogram because it's scaled as it's gathered by a potentially time-varying global number so the on-demand totalling might not give correct results. Also, in the presence of multiple thread pools any thread usage information should be per-pool not global. At the time I wrote this patch I concluded that I couldn't make the gathering scale and still preserve the exact semantics of the data gathered. > >> > > Yeah. The real issue here is deciding whether these stats are useful > or not; In my experience, not. > if not, can they be made useable? A different form of the data could certainly be made useful. -- Greg Banks, P.Engineer, SGI Australian Software Group. the brightly coloured sporks of revolution. I don't speak for SGI. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
