Good to know. What operations do you mostly do in the priority queue? I'd like to point you to the timer implementation that we did in the linux kernel ages ago. It is basically a set of log2 slots where each slot contains timers for time [2^i..2^(i+1)>. The dlinked lists make it O(1) to remove, change, or add a timer. The timer interrupt basically "eats" linked lists, and when empty "splats" the first free slot out over the upper slots (which is O(n) worst case). The structure is simple and optimized for doing a lot of mutation of the timers. It beats a heap hands down in the kernel at least. Alexander On Thu, Jan 31, 2013 at 10:34 AM, Edward Z. Yang wrote:

Hey Alex,

Thanks for CR. At the current point in time, the overall system design is very much in flux so a lot of this code may end up not existing in the final revision. I will be sure to up the number of comments in the final version though!

Edward

Excerpts from Alexander Kjeldaas's message of Thu Jan 31 01:30:39 -0800 2013:

...

Hi Edward, thanks for your work on the new scheduler!

I have a done a super light-weight review. I think documenting the code a little more would help readability.

http://hackage.haskell.org/trac/ghc/attachment/ticket/7606/0002-Stride-sched...

...

TSO.h:

Could you document ss_*? These are all important variables in the scheduler, and not documented. For example, the code in Threads.c for setting these is can act as some documentation, but something needs to be documented here.

(Please don't point to a paper as primary documentation.)

// 64-bit to prevent overflows; only ever accessed by the task which owns TSO. 170 StgWord64 ss_pass; 171 // These are bounded above by STRIDE1, which is less than max 32-bit word. 172 // You must take out the sched_lock to write to these; reads are OK 173 StgWord ss_tickets, ss_stride, ss_remain; 174

Schedule.c:

727 StgTSO *t;

Rename to 'tso' to be descriptive.

744 // go through all of the TSOs in the run queue and decide where 745 // they should go 746 // XXX We can create the new heap more efficiently O(n) by just 747 // blitting them in and then re-heapifying 748 if (!emptyRunQueue(cap)) { 749 StgWord64 k;

Ditto for 'k'.

1201 scheduleHandleThreadBlocked( Capability *cap, StgTSO *t )

Rename 't' to 'tso'.

Schedule.h:

125 // oh no magic constant 126 #define STRIDE1 (1 << 20)

Document STRIDE1

147 EXTERN_INLINE void 148 annulTSO(StgTSO *tso) { 149 // hack to make some invariants with regards to block_info and _link work 150 // this is called whereever we would have stepped all over the 151 // fields in the linked list implementation 152 tso->_link = END_TSO_QUEUE; 153 tso->block_info.closure = (StgClosure*)END_TSO_QUEUE;

It would be great to have a pointer to the invariants, or the

invariant(s)

...

documented.

213 tso->ss_pass += tso->ss_stride; 214 StgWord64 r; 215 if (tso->ss_pass <= cap->ss_pass) { 216 // Thread is behind, it will get scheduled in front with no 217 // intervention (note that cap->ss_pass is probably nonsense, 218 // since it doesn't include *this* thread.) 219 r = tso->ss_pass; 220 } else if (tso->ss_pass - tso->ss_pass <= cap->ss_pass) {

This expression looks weird/magic, tso->ss_pass - tso->ss_pass is 0.

221 // Thread is in good standing, schedule it in front 222 // (next iteration, they will not be in good standing if 223 // the global pass doesn't advance by much; that is, this 224 // thread managed to cut in front of other threads which 225 // are running behind.) 226 r = cap->ss_pass; 227 } else { 228 // Thread is not in good standing, schedule it later. 229 // Eventually, global pass will advance enough that the 230 // thread will be in good standing again, and can cut 231 // to the front. 232 r = tso->ss_pass;

Threads.c:

361 if (migrating) { 362 joinRunQueue(cap,tso); 363 } else { 364 appendToRunQueue(cap,tso); 365 }

Space after ','

Select.c: and rts/win32/AsyncIO.c

309 tso->ss_remain = 0; 310 joinRunQueue(&MainCapability,tso);

Should this be an abstraction in itself?

rts/Capability.h:

59 PQueue *run_pqueue; 60 61 // [SSS] Stride scheduling extensions. The Task with this 62 // Capability has exclusive access to this variable. 63 nat ss_pass;

Document ss_pass. For example how it relates to capPassUpdate and pushOnRunQueue (the weird/magic I commented on above) and invariants.

Alexander