In a typical N-Body simulation, at the end of each epoch, each locale would need to share its own portion of the world (i.e. all bodies) to the rest of the locales. I am working on this with a local-view approach (i.e. using on Loc statements). I encountered some strange behaviours that I couldn't make sense out of, so I decided to make a test program, in which things got more complicated. Here's the code to replicate the experiment.
proc log(args...?n) {
writeln("[locale = ", here.id, "] [", datetime.now(), "] => ", args);
}
const max: int = 50000;
record stuff {
var x1: int;
var x2: int;
proc init() {
this.x1 = here.id;
this.x2 = here.id;
}
}
class ctuff {
var x1: int;
var x2: int;
proc init() {
this.x1 = here.id;
this.x2 = here.id;
}
}
class wrapper {
// The point is that total size (in bytes) of data in `r`, `c` and `a` are the same here, because the record and the class hold two ints per index.
var r: [{1..max / 2}] stuff;
var c: [{1..max / 2}] owned ctuff?;
var a: [{1..max}] int;
proc init() {
this.a = here.id;
}
}
proc test() {
var wrappers: [LocaleSpace] owned wrapper?;
coforall loc in LocaleSpace {
on Locales[loc] {
wrappers[loc] = new owned wrapper();
}
}
// rest of the experiment further down.
}
Two interesting behaviours happen here.
1. Moving data
Now, each instance of wrapper in array wrappers should live in its locale. Specifically, the references (wrappers) will live in locale 0, but the internal data (r, c, a) should live in the respective locale. So we try to move some from locale 1 to locale 3, as such:
on Locales[3] {
var timer: Timer;
timer.start();
var local_stuff = wrappers[1]!.r;
timer.stop();
log("get r from 1", timer.elapsed());
log(local_stuff);
}
on Locales[3] {
var timer: Timer;
timer.start();
var local_c = wrappers[1]!.c;
timer.stop();
log("get c from 1", timer.elapsed());
}
on Locales[3] {
var timer: Timer;
timer.start();
var local_a = wrappers[1]!.a;
timer.stop();
log("get a from 1", timer.elapsed());
}
Surprisingly, my timings show that
Regardless of the size (const max), the time of sending the array and record strays constant, which doesn't make sense to me. I even checked with chplvis, and the size of GET actually increases, but the time stays the same.
The time to send the class field increases with time, which makes sense, but it is quite slow and I don't know which case to trust here.
2. Querying the locales directly.
To demystify the problem, I also query the .locale.id of some variables directly. First, we query the data, which we expect to live in locale 2, from locale 2:
on Locales[2] {
var wrappers_ref = wrappers[2]!; // This is always 1 GET from 0, okay.
log("array",
wrappers_ref.a.locale.id,
wrappers_ref.a[1].locale.id
);
log("record",
wrappers_ref.r.locale.id,
wrappers_ref.r[1].locale.id,
wrappers_ref.r[1].x1.locale.id,
);
log("class",
wrappers_ref.c.locale.id,
wrappers_ref.c[1]!.locale.id,
wrappers_ref.c[1]!.x1.locale.id
);
}
And the result is:
[locale = 2] [2020-12-26T19:36:26.834472] => (array, 2, 2)
[locale = 2] [2020-12-26T19:36:26.894779] => (record, 2, 2, 2)
[locale = 2] [2020-12-26T19:36:27.023112] => (class, 2, 2, 2)
Which is expected. Yet, if we query the locale of the same data on locale 1, then we get:
[locale = 1] [2020-12-26T19:34:28.509624] => (array, 2, 2)
[locale = 1] [2020-12-26T19:34:28.574125] => (record, 2, 2, 1)
[locale = 1] [2020-12-26T19:34:28.700481] => (class, 2, 2, 2)
Implying that wrappers_ref.r[1].x1.locale.id lives in locale 1, even though it should clearly be on locale 2. My only guess is that by the time .locale.id is executed, the data (i.e. the .x of the record) is already moved to the querying locale (1).
So all in all, the second part of the experiment lead to a secondary question, whilst not answering the first part.
NOTE: all experiment are run with -nl 4 in chapel/chapel-gasnet docker image.
Good observations, let me see if I can shed some light.
As an initial note, any timings taken with the gasnet Docker image should be taken with a grain of salt since that image simulates the execution across multiple nodes using your local system rather than running each locale on its own compute node as intended in Chapel. As a result, it is useful for developing distributed memory programs, but the performance characteristics are likely to be very different than running on an actual cluster or supercomputer. That said, it can still be useful for getting coarse timings (e.g., your "this is taking a much longer time" observation) or for counting communications using chplvis or the CommDiagnostics module.
With respect to your observations about timings, I also observe that the array-of-class case is much slower, and I believe I can explain some of the behaviors:
First, it's important to understand that any cross-node communications can be characterized using a formula like alpha + beta*length. Think of alpha as representing the basic cost of performing the communication, independent of length. This represents the cost of calling down through the software stack to get to the network, putting the data on the wire, receiving it on the other side, and getting it back up through the software stack to the application there. The precise value of alpha will depend on factors like the type of communication, choice of software stack, and physical hardware. Meanwhile, think of beta as representing the per-byte cost of the communication where, as you intuit, longer messages necessarily cost more because there's more data to put on the wire, or potentially to buffer or copy, depending on how the communication is implemented.
In my experience, the value of alpha typically dominates beta for most system configurations. That's not to say that it's free to do longer data transfers, but that the variance in execution time tends to be much smaller for longer vs. shorter transfers than it is for performing a single transfer versus many. As a result, when choosing between performing one transfer of n elements vs. n transfers of 1 element, you'll almost always want the former.
To investigate your timings, I bracketed your timed code portions with calls to the CommDiagnostics module as follows:
resetCommDiagnostics();
startCommDiagnostics();
...code to time here...
stopCommDiagnostics();
printCommDiagnosticsTable();
and found, as you did with chplvis, that the number of communications required to localize the array of records or array of ints was constant as I varied max, for example:
locale
get
execute_on
0
0
0
1
0
0
2
0
0
3
21
1
This is consistent with what I'd expect from the implementation: That for an array of value types, we perform a fixed number of communications to access array meta-data, and then communicate the array elements themselves in a single data transfer to amortize the overheads (avoid paying multiple alpha costs).
In contrast, I found that the number of communications for localizing the array of classes was proportional to the size of the array. For example, for the default value of 50,000 for max, I saw:
locale
get
put
execute_on
0
0
0
0
1
0
0
0
2
0
0
0
3
25040
25000
1
I believe the reason for this distinction relates to the fact that c is an array of owned classes, in which only a single class variable can "own" a given ctuff object at a time. As a result, when copying the elements of array c from one locale to another, you're not just copying raw data, as with the record and integer cases, but also performing an ownership transfer per element. This essentially requires setting the remote value to nil after copying its value to the local class variable. In our current implementation, this seems to be done using a remote get to copy the remote class value to the local one, followed by a remote put to set the remote value to nil, hence, we have a get and put per array element, resulting in O(n) communications rather than O(1) as in the previous cases. With additional effort, we could potentially have the compiler optimize this case, though I believe it will always be more expensive than the others due to the need to perform the ownership transfer.
I tested the hypothesis that owned classes were resulting in the additional overhead by changing your ctuff objects from being owned to unmanaged, which removes any ownership semantics from the implementation. When I do this, I see a constant number of communications, as in the value cases:
locale
get
execute_on
0
0
0
1
0
0
2
0
0
3
21
1
I believe this represents the fact that once the language has no need to manage the ownership of the class variables, it can simply transfer their pointer values in a single transfer again.
Beyond these performance notes, it's important to understand a key semantic difference between classes and records when choosing which to use. A class object is allocated on the heap, and a class variable is essentially a reference or pointer to that object. Thus, when a class variable is copied from one locale to another, only the pointer is copied, and the original object remains where it was (for better or worse). In contrast, a record variable represents the object itself, and can be thought of as being allocated "in place" (e.g., on the stack for a local variable). When a record variable is copied from one locale to the other, it's the object itself (i.e., the record's fields' values) which are copied, resulting in a new copy of the object itself. See this SO question for further details.
Moving on to your second observation, I believe that your interpretation is correct, and that this may be a bug in the implementation (I need to stew on it a bit more to be confident). Specifically, I think you're correct that what's happening is that wrappers_ref.r[1].x1 is being evaluated, with the result being stored in a local variable, and that the .locale.id query is being applied to the local variable storing the result rather than the original field. I tested this theory by taking a ref to the field and then printing locale.id of that ref, as follows:
ref x1loc = wrappers_ref.r[1].x1;
...wrappers_ref.c[1]!.x1.locale.id...
and that seemed to give the right result. I also looked at the generated code which seemed to indicate that our theories were correct. I don't believe that the implementation should behave this way, but need to think about it a bit more before being confident. If you'd like to open a bug against this on Chapel's GitHub issues page, for further discussion there, we'd appreciate that.
I have the typical setup for file io which works well with select like:
int retval = select(maxfd +1 , &read_set, &write_set, &error_set, 0); // timeout==0 -> endless
But now I have a situation where I want to loop and check on every cycle if one of the file selectors become ready. I do not want to start a separate thread for that! Is there something in posix/linux which can be used, hopefully with the same FD_SET like data structures which checks for file state without waiting for them?
Yes, I can set timeout for select to a minimal value, but I hope it can be done without that.
POSIX says:
To effect a poll, the timeout parameter should not be a null pointer, and should point to a zero-valued timespec structure.
So for your application, it should be sufficient to call select like this:
struct timeval zero = { 0, 0 };
int retval = select(maxfd +1 , &read_set, &write_set, &error_set, &zero);
Say ...
you have about 20 Thing
very often, you do a complex calculation running through a loop of say 1000 items. The end result is a varying number around 20 each time
you don't know how many there will be until you run through the whole loop
you then want to quickly (and of course elegantly!) access the result set in many places
for performance reasons you don't want to just make a new array each time. note that unfortunately there's a differing amount so you can't just reuse the same array trivially.
What about ...
var thingsBacking = [Thing](repeating: Thing(), count: 100) // hard limit!
var things: ArraySlice<Thing> = []
func fatCalculation() {
var pin: Int = 0
// happily, no need to clean-out thingsBacking
for c in .. some huge loop {
... only some of the items (roughly 20 say) become the result
x = .. one of the result items
thingsBacking[pin] = Thing(... x, y, z )
pin += 1
}
// and then, magic of slices ...
things = thingsBacking[0..<pin]
(Then, you can do this anywhere... for t in things { .. } )
What I am wondering, is there a way you can call to an ArraySlice<Thing> to do that in one step - to "append to" an ArraySlice and avoid having to bother setting the length at the end?
So, something like this ..
things = ... set it to zero length
things.quasiAppend(x)
things.quasiAppend(x2)
things.quasiAppend(x3)
With no further effort, things now has a length of three and indeed the three items are already in the backing array.
I'm particularly interested in performance here (unusually!)
Another approach,
var thingsBacking = [Thing?](repeating: Thing(), count: 100) // hard limit!
and just set the first one after your data to nil as an end-marker. Again, you don't have to waste time zeroing. But the end marker is a nuisance.
Is there a more better way to solve this particular type of array-performance problem?
Based on MartinR's comments, it would seem that for the problem
the data points are incoming and
you don't know how many there will be until the last one (always less than a limit) and
you're having to redo the whole thing at high Hz
It would seem to be best to just:
(1) set up the array
var ra = [Thing](repeating: Thing(), count: 100) // hard limit!
(2) at the start of each run,
.removeAll(keepingCapacity: true)
(3) just go ahead and .append each one.
(4) you don't have to especially mark the end or set a length once finished.
It seems it will indeed then use the same array backing. And it of course "increases the length" as it were each time you append - and you can iterate happily at any time.
Slices - get lost!
I would like a counter. The starting number will be:
10,000,000
Every 6 seconds, it will add 1, so it will be: 10,000,001 and then 10,000,002 and so on...
I would like to able to style the number: font-family, color, font-size, etc.
Can some please help me?
jQuery includes a function called setTimeout(), which causes a function to be called after a set time-delay. Something like the following would do what you are asking. Ensure that your document includes a DOM element with the id counter. Then:
var counter = 10000000;
function incrementCounter() {
counter++;
$('#counter').html(counter);
setTimeout(incrementCounter, 6000);
}
setTimeout(incrementCounter, 6000);
What’s going on here? setTimeout takes two arguments: the function to be called and the time-delay in milliseconds. The final line sets the function incrementCounter(), which we defined, to run after a delay of six seconds. The function increments the counter variable, sets the DOM object’s text to the value of the counter variable, then sets the timeout again: this means that the function will run every six seconds until something stops it.
As for styling the counter, this can be done either using static CSS or with the jQuery style-manipulation functions.
You can make use of setInterval to initiate a function which would be invoked in every 6000 milliseconds.
var num = 10000000;
setInterval(function()
{
num++;
console.log(num);
$('div').text(num.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ","));
},6000);
Here's an example : https://jsfiddle.net/DinoMyte/sac63azn/3/
Apologies if the question is poorly phrased, I'll do my best.
If I have a sequence of values with times as an Observable[(U,T)] where U is a value and T is a time-like type (or anything difference-able I suppose), how could I write an operator which is an auto-reset one-touch barrier, which is silent when abs(u_n - u_reset) < barrier, but spits out t_n - t_reset if the barrier is touched, at which point it also resets u_reset = u_n.
That is to say, the first value this operator receives becomes the baseline, and it emits nothing. Henceforth it monitors the values of the stream, and as soon as one of them is beyond the baseline value (above or below), it emits the elapsed time (measured by the timestamps of the events), and resets the baseline. These times then will be processed to form a high-frequency estimate of the volatility.
For reference, I am trying to write a volatility estimator outlined in http://www.amazon.com/Volatility-Trading-CD-ROM-Wiley/dp/0470181990 , where rather than measuring the standard deviation (deviations at regular homogeneous times), you repeatedly measure the time taken to breach a barrier for some fixed barrier amount.
Specifically, could this be written using existing operators? I'm a bit stuck on how the state would be reset, though maybe I need to make two nested operators, one which is one-shot and another which keeps creating that one-shot... I know it could be done by writing one by hand, but then I need to write my own publisher etc etc.
Thanks!
I don't fully understand the algorithm and your variables in the example, but you can use flatMap with some heap-state and return empty() or just() as needed:
int[] var1 = { 0 };
source.flatMap(v -> {
var1[0] += v;
if ((var1[0] & 1) == 0) {
return Observable.just(v);
}
return Observable.empty();
});
If you need a per-sequence state because of multiple consumers, you can defer the whole thing:
Observable.defer(() -> {
int[] var1 = { 0 };
return source.flatMap(v -> {
var1[0] += v;
if ((var1[0] & 1) == 0) {
return Observable.just(v);
}
return Observable.empty();
});
}).subscribe(...);