Thanks for doing a code review .. v helpful.
Simon
From: ghc-devs-bounces@haskell.org [mailto:ghc-devs-bounces@haskell.org]
On Behalf Of Alexander Kjeldaas
Sent: 31 January 2013 09:31
To: Edward Z. Yang; ghc-devs
Subject: Code review, new scheduler:
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.
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