AFS Performance. Simon Wilkinson
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1 AFS Performance Simon Wilkinson
2
3 Benchmarking Lots of different options for benchmarking rxperf - measures just RX layer performance filebench - generates workload against real server
4 The AFS Model Listener Service Listener Service
5
6 Single CPU
7 Kernel interrupts Listener Service Listener Service
8 Kernel interrupts Code runs at different interrupt priorities Cache manager at USER, RX at NET Masking interrupts provides mutual exclusion
9 LWP Listener Service Listener Service
10 LWP Multiplexes multiple threads into a single process Context switches are co-operative When a process elects to yield When a process sleeps or blocks When a process does I/O Locking need only be correct over a context switch
11
12 Multiple CPUs
13 LWP
14 pthreads Listener Listener Service Service
15 Global locks
16 Contention Global locks lead to high levels of contention Solution is to implement finer grained locks Initially a global lock per code module AFS_GLOCK RX (later split into fine grained locks) Vol Host
17 Finer grained locking Multiple per-data structure locks Protect individual structures against concurrent access For example, the rx_call lock
18 mutrace: Showing 10 mutexes in order of (write) contention count:! Mutex # Locked Changed Cont. cont.time[ms] tot.time[ms] avg.time[ms] Flags M M M M M M M M M M / Object: M = Mutex, W = RWLock / State: x = dead,! = inconsistent / Use: R = used in realtime thread / Mutex Type: r = RECURSIVE, e = ERRORCHECK, a = ADAPTIVE / Mutex Protocol: i = INHERIT, p = PROTECT / RWLock Kind: r = PREFER_READER, w = PREFER_WRITER, W = PREFER_WRITER_NONREC Mutex #16 (0x0x6b9100) first referenced by: rx.c:235 rxi_initpthread()! Mutex #2 (0x0x7f0df ) first referenced by: rx.c:2645 rxi_newcall()! Mutex #1 (0x0x7f0dd ) first referenced by: rx.c:2645 rxi_newcall()! Mutex #0 (0x0x7f0ddc000940) first referenced by: rx.c:2645 rxi_newcall()! Mutex #38 (0x0x7f0de ) first referenced by: rx.c:2645 rxi_newcall()! Mutex #12 (0x0x6b8ec0) first referenced by: rx.c:240 rxi_initpthread()! Mutex #36 (0x0x13d9da8) first referenced by: rx.c:1008 rx_newconnection()! Mutex #11 (0x0x6b8e40) first referenced by: rx.c:265 rxi_initpthread()! Mutex #37 (0x0x13da128) first referenced by: rx.c:2878 rxi_findpeer()! Mutex #26 (0x0x6ba480) first referenced by: rx_event.c:154 rxevent_init()
19 Detection tools plockstat (Mac OS X Mavericks, Solaris) mutrace (Linux)
20 False sharing CPUs have a small amount of level one cache Typically 64 bytes in length, page aligned Accessing data in the same cache line from multiple processors leads to cache thrashing
21 False sharing 64 bytes lock1 lock2 c1 c2
22 False sharing 2 64 bytes lock1 c1 lock2 lock2 c2
23 False sharing 3 64 bytes lock1 c1 lock2 c2
24 False sharing tools Sherriff DETECT and Sherriff PROTECT
25 Ad Hoc malloc Malloc was considered too expensive to use for every object Instead arrays of objects are constructed and managed locally Free object queues protected by a lock
26 Malloc issues Busy structures lead to high degree of contention on free list lock No thread locality for reused memory blocks False sharing between adjacent memory blocks
27 Cross Structure Sharing 64 bytes
28 malloc solutions Modern malloc can do better
29 Counters! MUTEX_ENTER(&counterLock);!! counter++;!! MUTEX_EXIT(&counterLock); Acquiring a lock is a heavy weight operation Blocking on a lock results in the thread being suspended Maintaining counters like this is very inefficient
30 Atomics! rx_atomic_inc(&counter); Atomic implementation available for most platforms i386 falls back to using mutexes Considerably faster, even in the uncontended case BUT heavily used counters can still suffer from cache thrashing
31 Per thread counters Maintain an individual counter in each thread Consolidate counters into a global value at regular intervals, or when the counter is queried Minimises impact of maintaining statistics counters
32 Lock step Reaction to lock contention - drop the lock more frequently! MUTEX_ENTER(&struct->lock);!! docalculationa(struct);!! docalculationb(struct);!! docalculationc(struct);!! MUTEX_EXIT(&struct->lock);! MUTEX_ENTER(&struct->lock);!! docalculationa(struct);!! MUTEX_EXIT(&struct->lock);!!! MUTEX_ENTER(&struct->lock);!! docalculationb(struct);!! MUTEX_EXIT(&struct->lock);!!! MUTEX_ENTER(&struct->lock);!! docalculationc(struct);!! MUTEX_EXIT(&struct->lock);
33 Lock step 2! MUTEX_ENTER(&struct->lock);!! docalculationa(struct);!! docalculationb(struct);!! docalculationc(struct);!! MUTEX_EXIT(&struct->lock);! MUTEX_ENTER(&struct->lock);!! docalculationa(struct);!! docalculationb(struct);!! docalculationc(struct);!! MUTEX_EXIT(&struct->lock);
34 Lock step 3! MUTEX_ENTER(&struct->lock);!! docalculationa(struct);!! MUTEX_EXIT(&struct->lock);!!! MUTEX_ENTER(&struct->lock);!! docalculationb(struct);!! MUTEX_EXIT(&struct->lock);!!! MUTEX_ENTER(&struct->lock);!! docalculationc(struct);!! MUTEX_EXIT(&struct->lock); x2
35 Lock step 4 Context switches means that total run time is longer (Latency for first thread less, though) Gets much worse as contention grows
36 Hot threads Removes context switches by running listener and service in same thread! Listener Service!! Listener Service Actually slows things down in a multi-core pthreaded world
37 Thundering Herds
38 Condition Variables Lots of threads waiting on a conditional variable Broadcast a wakeup on that variable But only one thread can continue Rest wake up, then sleep immediately afterwards
39 mutrace: Showing 10 mutexes in order of (write) contention count: Mutex # Locked Changed Cont. cont.time[ms] tot.time[ms] avg.time[ms] Flags M M Mx Mx Mx Mx Mx Mx Mx Mx / Object: M = Mutex, W = RWLock / State: x = dead,! = inconsistent / Use: R = used in realtime thread / Mutex Type: r = RECURSIVE, e = ERRORCHECK, a = ADAPTIVE / Mutex Protocol: i = INHERIT, p = PROTECT / RWLock Kind: r = PREFER_READER, w = PREFER_WRITER, W = PREFER_WRITER_NONREC
40 OpenAFS 1.4 OpenAFS 1.6 OpenAFS master Current YFS Time (in ms) to copy 60Mbytes of data over RX
41 Configuration Changes Experiment with schedulers Turn off hot threads Set CPU affinity to only use cores on the same die
42 Questions?
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