On Tue, Sep 24, 2019 at 12:01 AM Bishop Bettini <bis...@php.net> wrote: > > On Tue, Sep 24, 2019 at 2:26 AM Peter Stalman <sarke...@gmail.com> wrote: >> >> When PHP runs out of memory, a fatal error is triggered and whatever shutdown >> functions or error handlers take over. >> >> However, in the case of error logging, or just logging in general, there >> often >> needs to be additional memory used to accommodate the final logging process. >> >> This can sort of be accomplished in userland a few ways: >> >> 1. Pre-allocating memory in a variable, such as the Yii2 error handler >> (http://bit.ly/2kLnpd2), but this requires wasting memory on every request. >> >> 2. Continuously checking the memory usage, but this increases code complexity >> needlessly and also wastes resources with constant checking. >> >> 3. A second process with its own memory allowance, but this also increases >> complexity and transferring the required data for logging would require >> serialization without using additional memory. I'm not sure how this would >> be >> accomplished. >> >> So I would like to suggest an option for setting a shutdown memory allowance, >> which would be the amount of additional memory allowed to be used by any >> registered error handlers or shutdown functions. >> >> I think a C implementation of this in PHP would be far more efficient than >> the >> userland implementionations I mentioned. > > > Memory parachutes help when against a hard limit, but I'm not sure they're > applicable in the soft limit scenario you describe. When the OOM condition > fires, PHP resets its soft tracking of memory. You are free to allocate as > much on the stack of the shutdown function, up to the soft limit again. > > Here[1] we see the shutdown function called because of OOM, then we allocate > a local variable that consumes less than the soft limit (which is ok), while > here[2] we see the shutdown function itself is constrained to the soft limit > (it's killed b/c of OOM). > > Perhaps I am misunderstanding the scenario. Could you elaborate further, > perhaps provide a concrete example demonstrating where a parachute would be > needed? > > [1]:https://3v4l.org/bXMlX > [2]:https://3v4l.org/HDkRc
Hi Bishop, Thanks for your reply. However, I do not think your statement that the memory limit resets is correct. At least, not in a practical real-world sense, as the logger (or perhaps other shutdown functions as well) need to be able to access Your examples do not really work for a few reasons. I don't think 2048 is a valid value for the memory limit, as the output in your examples show it is 2 mb (which I believe is the minimum). The docs also state the number is in bytes. Even so, this is not a real-world number, and the way PHP handles it's memory I don't think using only one variable will give us a real-world example either. As you can see from your first example, PHP tries to assign an additional 2 mb just to add another element to the array. I believe there is dome doubling going on with the memory allocation, when increasing the array size, so it doesn't have to be done on every append. Since that additional assignment failed, the actual memory usage goes back to what it was before the assignment, which means any subsequent memory usage can be up to the 2 mb that you tried to assign. That's why the smaller loop in the shutdown function works, as the original variable is probably only about 1 mb in size. The fact that your second example tries to assign only 516 kb when it fails at the memory limit supports this, and also suggests that that array was much smaller when it hit the limit. If it could assign as much as the original, then it would have tried to increase the memory allocation by about the same amount. I've put together my own example as you suggested, using an array of strings instead. This would reduce the incremental memory allocations as there is no longer a single giant variable being doubled. It fails when trying to assign 416 kb. https://3v4l.org/BqfvU If you are correct then the shutdown function should be able to recreate the original array and not run out of memory. However, as you can see, it fails after only assigning 2 of the 100 elements of the original array, which suggests the shutdown function is using the same memory and it's limit. Thanks, Peter -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: http://www.php.net/unsub.php
