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digitalmars.D - try...catch slooowness?

reply spir <denis.spir gmail.com> writes:
Hello,


I had not initially noticed that the 'in' operator (for AAs) returns a poin=
ter to the looked up element. So that, to avoid double lookup in cases wher=
e lookups may fail, I naively used try...catch. In cases of very numerous l=
ookups, my code suddenly became blitz fast. So that I wondered about except=
ion handling efficiency. Below a test case (on my computer, both loops run =
in about the same average time):

void main () {
    byte[uint] table =3D [3:1, 33:1, 333:1];
    byte b;
    byte* p;
    Time t0;
    uint N1 =3D 246, N2 =3D 9999999;
   =20
    // try...catch
    t0 =3D time();
    foreach (n ; 0..N1) {
        try b =3D table[n];
        catch (RangeError e) {}
    }
    writefln("try...catch version time: %sms", time() - t0);
   =20
    // pointer
    t0 =3D time();
    foreach (n ; 0..N2) {
        p =3D (n in table);
        if (p) b =3D table[n];
    }
    writefln("pointer version time: %sms", time() - t0);
   =20
    writefln("pointer version is about %s times faster",N2/N1);
}
=3D=3D>
try...catch version time: 387ms
pointer version time: 388ms
pointer version is about 40650 times faster

Note that both versions perform a single lookup trial; the difference thus =
only lies in pointer deref vs try...catch handling, i guess. What do you th=
ink?


Denis
-- -- -- -- -- -- --
vit esse estrany =E2=98=A3

spir.wikidot.com
Dec 19 2010
next sibling parent reply bearophile <bearophileHUGS lycos.com> writes:
spir:

 try...catch version time: 387ms
 pointer version time: 388ms
 pointer version is about 40650 times faster

Those numbers look wrong :-)
 What do you think?

Compared to Oracle Java VM the DMD exceptions are very slow. Performance tuning of DMD is left for later, when the main features are all present. Bye, bearophile
Dec 19 2010
parent reply Michel Fortin <michel.fortin michelf.com> writes:
On 2010-12-19 08:17:07 -0500, spir <denis.spir gmail.com> said:

 On Sun, 19 Dec 2010 07:46:09 -0500
 bearophile <bearophileHUGS lycos.com> wrote:
 
 spir:
 
 try...catch version time: 387ms
 pointer version time: 388ms
 pointer version is about 40650 times faster

Those numbers look wrong :-)

I thought so. But in the real app a loop that lasted ~ 10s suddenly became instantaneous (human perception ;-) Try it. Sorry, but I'm not inventing the numbers.

What looks wrong is that the output seem to say that 388ms was 40650 times faster than 387ms. Obviously, by looking at the code one can understand that you adjusted the number of iteration to get the same time for both versions, and from that number of iteration you can claim the pointer version is about X times faster. But the output alone is misleading. Add the iteration count to the output and it'll be easier to read. Exceptions are slow, that's a fact of life. The idea is that an exception should be exceptional, so the case to optimize for is the case where you don't have any exception: a try...catch that doesn't throw. Other ways to implement exceptions exists which are faster at throwing (setjmp for instance), but they're also slower at entering and exiting a try..catch block when no exception occur.
 What do you think?

Compared to Oracle Java VM the DMD exceptions are very slow. Performance tuning of DMD is left for later, when the main features are all present.

And what if the numbers are correct? Exception handling is explicitely reco mmanded in some docs --as opposed to C-like "manual" hacks.

Exceptions are recommended to avoid cluttering your normal code flow with error handling code. Clearly, in the code above exceptions are part of the normal code flow. That's not what exception are made for. That said, I'd encourage you to profile this test case to see where the time is being spent. You might find one or two places that can be improved in the part of the runtime dedicated to exceptions. -- Michel Fortin michel.fortin michelf.com http://michelf.com/
Dec 19 2010
parent reply Walter Bright <newshound2 digitalmars.com> writes:
Michel Fortin wrote:
 Exceptions are slow, that's a fact of life. The idea is that an 
 exception should be exceptional, so the case to optimize for is the case 
 where you don't have any exception: a try...catch that doesn't throw. 
 Other ways to implement exceptions exists which are faster at throwing 
 (setjmp for instance), but they're also slower at entering and exiting a 
 try..catch block when no exception occur.

[...]
 Exceptions are recommended to avoid cluttering your normal code flow 
 with error handling code. Clearly, in the code above exceptions are part 
 of the normal code flow. That's not what exception are made for.

Right on all counts. Exceptions are for *exceptional* cases, i.e. unexpected errors, not normal control flow. The implementation is designed so that the speed normal execution is strongly favored over speed of exception handling.
Dec 20 2010
parent reply "Rob" <rob2970 yah00.com> writes:
Walter Bright wrote:
 Michel Fortin wrote:
 Exceptions are slow, that's a fact of life. The idea is that an
 exception should be exceptional, so the case to optimize for is the
 case where you don't have any exception: a try...catch that doesn't
 throw. Other ways to implement exceptions exists which are faster at
 throwing (setjmp for instance), but they're also slower at entering
 and exiting a try..catch block when no exception occur.

[...]
 Exceptions are recommended to avoid cluttering your normal code flow
 with error handling code. Clearly, in the code above exceptions are
 part of the normal code flow. That's not what exception are made for.

Right on all counts. Exceptions are for *exceptional* cases, i.e. unexpected errors, not normal control flow.

Exceptions (actual ones and not just a reference to any particular error handling mechanism) are EXPECTED errors. That follows directly from the goal of keeping programs running by handling of errors that were thought about at design time. Show me an unexpected error and I'll show you a bug.
 The implementation is designed so that the speed normal execution is
 strongly favored over speed of exception handling. 

Dec 21 2010
parent "Rob" <rob2970 yah00.com> writes:
Steven Schveighoffer wrote:
 On Tue, 21 Dec 2010 17:16:57 -0500, Rob <rob2970 yah00.com> wrote:

 Walter Bright wrote:
 Michel Fortin wrote:
 Exceptions are slow, that's a fact of life. The idea is that an
 exception should be exceptional, so the case to optimize for is the
 case where you don't have any exception: a try...catch that doesn't
 throw. Other ways to implement exceptions exists which are faster
 at throwing (setjmp for instance), but they're also slower at
 entering and exiting a try..catch block when no exception occur.

[...]
 Exceptions are recommended to avoid cluttering your normal code
 flow with error handling code. Clearly, in the code above
 exceptions are part of the normal code flow. That's not what
 exception are made for.

Right on all counts. Exceptions are for *exceptional* cases, i.e. unexpected errors, not normal control flow.

Exceptions (actual ones and not just a reference to any particular error handling mechanism) are EXPECTED errors. That follows directly from the goal of keeping programs running by handling of errors that were thought about at design time. Show me an unexpected error and I'll show you a bug.

No they aren't. They are anticipated,

That is how the term "expected" is used in the literature. Read up. No need to play semantical/literal games. It's the concept that is relevant. The point is that saying that exceptions are unexpected is incorrect by just about any paper, article or book on error handling, save for those that get that wrong also (A. Andrescu's early book gets it wrong also, as I recall, FWIW).
Dec 22 2010
prev sibling next sibling parent Michel Fortin <michel.fortin michelf.com> writes:
On 2010-12-19 07:33:29 -0500, spir <denis.spir gmail.com> said:

     // pointer
     t0 = time();
     foreach (n ; 0..N2) {
         p = (n in table);
         if (p) b = table[n];
     }

But why the double lookup? Just dereference the pointer: foreach (n ; 0..N2) { p = (n in table); if (p) b = *p; } -- Michel Fortin michel.fortin michelf.com http://michelf.com/
Dec 19 2010
prev sibling next sibling parent spir <denis.spir gmail.com> writes:
On Sun, 19 Dec 2010 07:47:11 -0500
Michel Fortin <michel.fortin michelf.com> wrote:

 On 2010-12-19 07:33:29 -0500, spir <denis.spir gmail.com> said:
=20
     // pointer
     t0 =3D time();
     foreach (n ; 0..N2) {
         p =3D (n in table);
         if (p) b =3D table[n];
     }

But why the double lookup? Just dereference the pointer: =20 foreach (n ; 0..N2) { p =3D (n in table); if (p) b =3D *p; }

Oops, sorry, code copy error. But this does not change the test result time= s (because nearly no key exists in the table). denis -- -- -- -- -- -- -- vit esse estrany =E2=98=A3 spir.wikidot.com
Dec 19 2010
prev sibling next sibling parent spir <denis.spir gmail.com> writes:
On Sun, 19 Dec 2010 07:46:09 -0500
bearophile <bearophileHUGS lycos.com> wrote:

 spir:
=20
 try...catch version time: 387ms
 pointer version time: 388ms
 pointer version is about 40650 times faster

Those numbers look wrong :-)

I thought so. But in the real app a loop that lasted ~ 10s suddenly became = instantaneous (human perception ;-) Try it. Sorry, but I'm not inventing the numbers.
 What do you think?

Compared to Oracle Java VM the DMD exceptions are very slow. Performance =

And what if the numbers are correct? Exception handling is explicitely reco= mmanded in some docs --as opposed to C-like "manual" hacks. Denis -- -- -- -- -- -- -- vit esse estrany =E2=98=A3 spir.wikidot.com
Dec 19 2010
prev sibling next sibling parent Lutger Blijdestijn <lutger.blijdestijn gmail.com> writes:
spir wrote:

 Hello,
 
 
 I had not initially noticed that the 'in' operator (for AAs) returns a
 pointer to the looked up element. So that, to avoid double lookup in cases
 where lookups may fail, I naively used try...catch. In cases of very
 numerous lookups, my code suddenly became blitz fast. So that I wondered
 about exception handling efficiency. Below a test case (on my computer,
 both loops run in about the same average time):
 

Catching exceptions is known to be a bit costly. Generally, in C family languages with exceptions (such as D), it is considered bad practice to use exceptions for regular control flow. The cost is therefore acceptable because it is not suffered on the normal, happy code path. That the performance cost is so huge is likely not only the inherent overhead of exceptions, but probably also some extra cache misses or something like that.[needs analysis]
Dec 19 2010
prev sibling next sibling parent reply "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Sun, 19 Dec 2010 07:33:29 -0500, spir <denis.spir gmail.com> wrote:

 Hello,


 I had not initially noticed that the 'in' operator (for AAs) returns a  
 pointer to the looked up element. So that, to avoid double lookup in  
 cases where lookups may fail, I naively used try...catch. In cases of  
 very numerous lookups, my code suddenly became blitz fast. So that I  
 wondered about exception handling efficiency. Below a test case (on my  
 computer, both loops run in about the same average time):

 void main () {
     byte[uint] table = [3:1, 33:1, 333:1];
     byte b;
     byte* p;
     Time t0;
     uint N1 = 246, N2 = 9999999;
    // try...catch
     t0 = time();
     foreach (n ; 0..N1) {
         try b = table[n];
         catch (RangeError e) {}
     }
     writefln("try...catch version time: %sms", time() - t0);
    // pointer
     t0 = time();
     foreach (n ; 0..N2) {
         p = (n in table);
         if (p) b = table[n];
     }
     writefln("pointer version time: %sms", time() - t0);
    writefln("pointer version is about %s times faster",N2/N1);
 }
 ==>
 try...catch version time: 387ms
 pointer version time: 388ms
 pointer version is about 40650 times faster

 Note that both versions perform a single lookup trial; the difference  
 thus only lies in pointer deref vs try...catch handling, i guess. What  
 do you think?

This example is misleading. First, catching an exception should be a rare occurrence (literally, an exception to the rule). You are testing the case where catching an exception vastly outweighs the cases where an exception is not thrown. What I'm saying is, catching an exception is very slow, but *trying* to catch an exception is not. Second, exception handling is not meant to be used in the way you used it. You don't use it as an extra return value. I'd expect a more reasonable use of catching an exception in AAs as this: try { foreach(n ; 0..N1) { b = table[n]; } } catch(RangeError e) { writeln("Caught exception! ", e); } An exception is a recoverable error, but it usually means something is wrong, not 'business as usual'. This doesn't mean it's impossible to design poor interfaces that use exceptions for everything, but it shouldn't be that way. An exception should always be a rare occurrence, when something happens that you don't expect. A huge clue that you are using exceptions poorly or that the interface is not meant to be used that way is if your exception handling is being done at the innermost level of your program. Exception handling is great when it exists at a much higher level, because you can essentially do all error handling in one spot, and simply write code without worrying about error codes. This is why the 'in' operator exists for AAs. General rules of thumb for AAs: 1. if you expect that a value is always going to be present when you ask for it, use exception handling at a high level. 2. if you *don't* expect that, and want to check the existence of an element, use 'in' Now, after saying all that, improving how exception handling works can only be good. So comparing exception handling performance in D to exception handling in other languages can give a better idea of how well D's exception handling performs. -Steve
Dec 20 2010
parent reply "Rob" <rob2970 yah00.com> writes:
Steven Schveighoffer wrote:

 An exception is a recoverable error,

Not necessarily. At some point, all the handling options could have been tried but all failed in which case there is nothing left to do except for letting something higher up (like the operating system) deal with the situation. In such a case, recovery did not occur if you consider recovery to mean that the program keeps running normally.
 Exception handling is
 great when it exists at a much higher level, because you can
 essentially do all error handling in one spot, and simply write code
 without worrying about error codes.

That sounds like the common misconception that leads to weak designs.
Dec 21 2010
parent "Rob" <rob2970 yah00.com> writes:
Steven Schveighoffer wrote:
 On Tue, 21 Dec 2010 17:25:09 -0500, Rob <rob2970 yah00.com> wrote:

 Steven Schveighoffer wrote:

 An exception is a recoverable error,

Not necessarily. At some point, all the handling options could have been tried but all failed in which case there is nothing left to do except for letting something higher up (like the operating system) deal with the situation. In such a case, recovery did not occur if you consider recovery to mean that the program keeps running normally.

In D, unrecoverable errors derive from Error, recoverable ones derive from Exception. By 'recovery' I mean that the program can either continue to run or decide proactively to do something different (like print an error and exit).

That, decidedly, from common terminology usage industry-wide, is considered "unrecoverable". "Unrecoverable" doesn't mean crash and burn. Anything less than the program staying running is indicates something unrecoverable happened.
Dec 22 2010
prev sibling next sibling parent spir <denis.spir gmail.com> writes:
On Mon, 20 Dec 2010 12:29:29 -0500
"Steven Schveighoffer" <schveiguy yahoo.com> wrote:

 This example is misleading.  First, catching an exception should be a rar=

 occurrence (literally, an exception to the rule).  You are testing the =20
 case where catching an exception vastly outweighs the cases where an =20
 exception is not thrown.  What I'm saying is, catching an exception is =20
 very slow, but *trying* to catch an exception is not.

Right, understood, thank you. Denis -- -- -- -- -- -- -- vit esse estrany =E2=98=A3 spir.wikidot.com
Dec 20 2010
prev sibling next sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Tue, 21 Dec 2010 17:16:57 -0500, Rob <rob2970 yah00.com> wrote:

 Walter Bright wrote:
 Michel Fortin wrote:
 Exceptions are slow, that's a fact of life. The idea is that an
 exception should be exceptional, so the case to optimize for is the
 case where you don't have any exception: a try...catch that doesn't
 throw. Other ways to implement exceptions exists which are faster at
 throwing (setjmp for instance), but they're also slower at entering
 and exiting a try..catch block when no exception occur.

[...]
 Exceptions are recommended to avoid cluttering your normal code flow
 with error handling code. Clearly, in the code above exceptions are
 part of the normal code flow. That's not what exception are made for.

Right on all counts. Exceptions are for *exceptional* cases, i.e. unexpected errors, not normal control flow.

Exceptions (actual ones and not just a reference to any particular error handling mechanism) are EXPECTED errors. That follows directly from the goal of keeping programs running by handling of errors that were thought about at design time. Show me an unexpected error and I'll show you a bug.

No they aren't. They are anticipated, but not expected. For instance, an I/O error is not *expected*, but it is anticipated as possible if say the config file is not readable, but it's not expected to happen for a normal operation. In this case, the exception most likely kills the program (can't do much without a config), but the program handles the error gracefully and prints a readable message rather than just "Broken Pipe" or something like that. -Steve
Dec 22 2010
prev sibling next sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Tue, 21 Dec 2010 17:25:09 -0500, Rob <rob2970 yah00.com> wrote:

 Steven Schveighoffer wrote:

 An exception is a recoverable error,

Not necessarily. At some point, all the handling options could have been tried but all failed in which case there is nothing left to do except for letting something higher up (like the operating system) deal with the situation. In such a case, recovery did not occur if you consider recovery to mean that the program keeps running normally.

In D, unrecoverable errors derive from Error, recoverable ones derive from Exception. By 'recovery' I mean that the program can either continue to run or decide proactively to do something different (like print an error and exit). But control is still in the programmer's hands.
 Exception handling is
 great when it exists at a much higher level, because you can
 essentially do all error handling in one spot, and simply write code
 without worrying about error codes.

That sounds like the common misconception that leads to weak designs.

So you tell me, what is a good design with exceptions? Because it seems like doing: try fn() catch(Exception) {...} is pretty ugly and cumbersome (and clearly has performance issues), when it could be: if(fn() != 0) ... To me, if you are surrounding a single low-level call with an exception handler, your design has issues. -Steve
Dec 22 2010
prev sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Wed, 22 Dec 2010 15:21:52 -0500, Rob <rob2970 yah00.com> wrote:

 Steven Schveighoffer wrote:
 On Tue, 21 Dec 2010 17:16:57 -0500, Rob <rob2970 yah00.com> wrote:

 Walter Bright wrote:
 Michel Fortin wrote:
 Exceptions are slow, that's a fact of life. The idea is that an
 exception should be exceptional, so the case to optimize for is the
 case where you don't have any exception: a try...catch that doesn't
 throw. Other ways to implement exceptions exists which are faster
 at throwing (setjmp for instance), but they're also slower at
 entering and exiting a try..catch block when no exception occur.

[...]
 Exceptions are recommended to avoid cluttering your normal code
 flow with error handling code. Clearly, in the code above
 exceptions are part of the normal code flow. That's not what
 exception are made for.

Right on all counts. Exceptions are for *exceptional* cases, i.e. unexpected errors, not normal control flow.

Exceptions (actual ones and not just a reference to any particular error handling mechanism) are EXPECTED errors. That follows directly from the goal of keeping programs running by handling of errors that were thought about at design time. Show me an unexpected error and I'll show you a bug.

No they aren't. They are anticipated,

That is how the term "expected" is used in the literature. Read up. No need to play semantical/literal games. It's the concept that is relevant. The point is that saying that exceptions are unexpected is incorrect by just about any paper, article or book on error handling, save for those that get that wrong also (A. Andrescu's early book gets it wrong also, as I recall, FWIW).

Sorry, I guess I dummer than you. -Steve
Dec 22 2010