Re: [PHP-DEV] Safe timeout handling

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  92559
April 20, 2016 17:40 bobwei9@hotmail.com (Bob Weinand)
> Am 20.04.2016 um 18:22 schrieb Dmitry Stogov <dmitry@zend.com>: > > > > On 04/20/2016 06:24 PM, Matt Wilmas wrote: >> Hi Dmitry, >> >> ----- Original Message ----- >> From: "Dmitry Stogov" >> Sent: Wednesday, April 20, 2016 >> >>> Hi, >>> >>> >>> It's a well known PHP problem, that exceeding of execution time-out >>> (max_execution_time) may lead to unexpected crashes. >>> >>> They occur because PHP may be interrupted in inconsistent state, and attempt >>> to release allocated by request resources leads to failure. >>> >>> Almost any big site sees these crashes from time to time. >>> >>> >>> I propose to delay actual request termination until a "safe" point in >>> interpreter. >>> >>> Signal handler will just set EG(timed_out) flag. >>> >>> Interpreter will check this time from time to time (on jumps and calls that >>> may make loops or recursion) and perform the actual termination. >>> >>> This approach already works in PHP for Windows. >> >> I was thinking about this, checking for things like EG(exception) "constantly," a few months ago for another reason... > > This is a bit different problem. We can't delay EG(exception) checks. I thought about a better way of exception handing but didn't find anything usable and portable. >> >> What about instead of adding additional checks in the same place(s) in VM, we just limit it to 1 check, for multiple things? Just have EG(something_unexpected_to_check), and behind that (or in a function, I guess), the actual rare/unexpected thing gets checked: timed_out, exception, etc. > > Yes, I have the same idea in background. I even wrote: The same "interrupt" handling mechanism in the future may be reused for TICK > and signal handling. > >> >> It seems Bob had a similar idea in the PR comment, except literally using exceptions... >> >>> In addition I introduce hard_timeout (default value 2 seconds). >>> >>> In case the "soft" timeout wasn't handled "safely" in that 2 seconds >>> (because of long running internal function), PHP process will be terminated >>> without attempt to free any resources. >>> >>> ZTS build will ignore "hard_timeout" (in the same way as PHP on Windows do). >>> >>> >>> The PR: https://github.com/php/php-src/pull/1876 >>> >>> >>> It removes "exit_on_timeout" ini directive, and introduces "hard_timeout" >>> instead. >>> >>> Additional checks in VM make 0.5-1% slowdown in term of instruction retired >>> reported by callgrind. >> >> A single check would save those additional instructions and branches, and would actually improve things on Windows (since this PR doesn't change anything there). > > If you or Bob show me a better working solution (or just PoC), I'll be only happy with this.
I looked at it; if we had an already existing if (EG(exception)) branch there, we could actually save something, but Dmitry put it just in jumping ops, where we never have an exception check. (And there we need a check as else a while(1) {} would never be timed out - i.e. when there are no ops inside which actually do a check_exception) … so this EG(something_unexpected_to_check) is even more expensive. Things would be easy if we could just alter the return addresses of the opcode handlers or similar magic, but I doubt that's a very nice cross-platform solution... If you have an even better idea than I do… please show a PoC :-) Thanks, Bob
> Thanks. Dmitry. >> >> >>> I think we don't need RFC for this. This is a long time desired fix. >>> >>> >>> The same "interrupt" handling mechanism in the future may be reused for TICK >>> and signal handling. >>> >>> >>> Thanks. Dmitry. >> >> - Matt
  92562
April 20, 2016 18:06 dmitry@zend.com (Dmitry Stogov)
On 04/20/2016 08:40 PM, Bob Weinand wrote:
> >> Am 20.04.2016 um 18:22 schrieb Dmitry Stogov <dmitry@zend.com >> <mailto:dmitry@zend.com>>: >> >> >> >> On 04/20/2016 06:24 PM, Matt Wilmas wrote: >>> Hi Dmitry, >>> >>> ----- Original Message ----- >>> From: "Dmitry Stogov" >>> Sent: Wednesday, April 20, 2016 >>> >>>> Hi, >>>> >>>> >>>> It's a well known PHP problem, that exceeding of execution time-out >>>> (max_execution_time) may lead to unexpected crashes. >>>> >>>> They occur because PHP may be interrupted in inconsistent state, >>>> and attempt >>>> to release allocated by request resources leads to failure. >>>> >>>> Almost any big site sees these crashes from time to time. >>>> >>>> >>>> I propose to delay actual request termination until a "safe" point in >>>> interpreter. >>>> >>>> Signal handler will just set EG(timed_out) flag. >>>> >>>> Interpreter will check this time from time to time (on jumps and >>>> calls that >>>> may make loops or recursion) and perform the actual termination. >>>> >>>> This approach already works in PHP for Windows. >>> >>> I was thinking about this, checking for things like EG(exception) >>> "constantly," a few months ago for another reason... >> >> This is a bit different problem. We can't delay EG(exception) checks. >> I thought about a better way of exception handing but didn't find >> anything usable and portable. >>> >>> What about instead of adding additional checks in the same place(s) >>> in VM, we just limit it to 1 check, for multiple things? Just have >>> EG(something_unexpected_to_check), and behind that (or in a >>> function, I guess), the actual rare/unexpected thing gets checked: >>> timed_out, exception, etc. >> >> Yes, I have the same idea in background. I even wrote: The same >> "interrupt" handling mechanism in the future may be reused for TICK >> and signal handling. >> >>> >>> It seems Bob had a similar idea in the PR comment, except literally >>> using exceptions... >>> >>>> In addition I introduce hard_timeout (default value 2 seconds). >>>> >>>> In case the "soft" timeout wasn't handled "safely" in that 2 seconds >>>> (because of long running internal function), PHP process will be >>>> terminated >>>> without attempt to free any resources. >>>> >>>> ZTS build will ignore "hard_timeout" (in the same way as PHP on >>>> Windows do). >>>> >>>> >>>> The PR: https://github.com/php/php-src/pull/1876 >>>> >>>> >>>> It removes "exit_on_timeout" ini directive, and introduces >>>> "hard_timeout" >>>> instead. >>>> >>>> Additional checks in VM make 0.5-1% slowdown in term of instruction >>>> retired >>>> reported by callgrind. >>> >>> A single check would save those additional instructions and >>> branches, and would actually improve things on Windows (since this >>> PR doesn't change anything there). >> >> If you or Bob show me a better working solution (or just PoC), I'll >> be only happy with this. > > I looked at it; if we had an already existing if (EG(exception)) > branch there, we could actually save something, but Dmitry put it just > in jumping ops, where we never have an exception check. (And there we > need a check as else a while(1) {} would never be timed out - i.e. > when there are no ops inside which actually do a check_exception) … so > this EG(something_unexpected_to_check) is even more expensive. I meant later we may replace check for EG(timed_out) with check for
EG(something_unexpected_to_check), and then perform additional checks in zend_vm_interrupt(). Thanks. Dmitry.
> > Things would be easy if we could just alter the return addresses of > the opcode handlers or similar magic, but I doubt that's a very nice > cross-platform solution... > > If you have an even better idea than I do… please show a PoC :-) > > Thanks, > Bob > >> Thanks. Dmitry. >>> >>> >>>> I think we don't need RFC for this. This is a long time desired fix. >>>> >>>> >>>> The same "interrupt" handling mechanism in the future may be reused >>>> for TICK >>>> and signal handling. >>>> >>>> >>>> Thanks. Dmitry. >>> >>> - Matt >