Update (2016-06-14): This article is about memory usage in PHP 5. Memory usage in PHP 7 is, for the case covered here, approximately three times lower. For more information, see my article on the hashtable implementation in PHP 7.
Upfront I want to thank Johannes and Tyrael for their help in finding some of the more hidden memory usage.
In this post I want to investigate the memory usage of PHP arrays (and values in general) using the following script as an example, which creates 100000 unique integer array elements and measures the resulting memory usage:
How much would you expect it to be? Simple, one integer is 8 bytes (on a 64 bit unix machine and
long type) and you got 100000 integers, so you obviously will need 800000 bytes.
That’s something like 0.76 MBs.
Now try and run the above code. This gives me
14649024 bytes. Yes, you heard right, that’s 13.97 MB - eighteen times
more than we estimated.
So, where does that extra factor of 18 come from?
For those who don’t want to know the full story, here is a quick summary of the memory usage of the different components involved:
| 64 bit | 32 bit --------------------------------------------------- zval | 24 bytes | 16 bytes + cyclic GC info | 8 bytes | 4 bytes + allocation header | 16 bytes | 8 bytes =================================================== zval (value) total | 48 bytes | 28 bytes =================================================== bucket | 72 bytes | 36 bytes + allocation header | 16 bytes | 8 bytes + pointer | 8 bytes | 4 bytes =================================================== bucket (array element) total | 96 bytes | 48 bytes =================================================== total total | 144 bytes | 76 bytes
The above numbers will vary depending on your operating system, your compiler and your compile options. E.g. if you compile PHP with debug or with thread-safety, you will get different numbers. But I think that the sizes given above are what you will see on an average 64-bit production build of PHP 5.3 on Linux.
If you multiply those 144 bytes by our 100000 elements you get 14400000 bytes, which is 13.73 MB. That’s pretty close to the real number - the rest is mostly pointers for uninitialized buckets, but I’ll cover that later.
Now, if you want to have a more detailed analysis of the values mentioned above, read on :)
First have a look at how PHP stores values. As you know PHP is a weakly typed language, so it needs
some way to switch between the various types fast. PHP uses a
union for this, which is
defined as follows in
zend.h#307 (comments mine):
If you don’t know C, that isn’t a problem as the code is pretty straightforward: A
union is a
means to make some value accessible as various types. For example if you do a
you’ll get the value interpreted as an integer. If you use
zvalue_value->ht on the other hand the
value will be interpreted as a pointer to a hashtable (aka array).
But let’s not get too much into this here. Important for us only is that the size of a
equals the size of its largest component. The largest component here is the string struct (the
zend_object_value struct has the same size as the
str struct, but I’ll leave that out for
simplicity). The string struct stores a pointer (8 bytes) and an integer (4 bytes), which is 12
bytes in total. Due to memory alignment (structs with 12 bytes aren’t cool because they aren’t a
multiple of 64 bits / 8 bytes) the total size of the struct will be 16 bytes though and that will
also be the size of the union as a whole.
So now we know that we don’t need 8 bytes for every value, but 16 - due to PHP’s dynamic typing. Multiplying by 100000 values gives us 1600000 bytes, i.e. 1.53 MB. But the real value is 13.97 MB, so we can’t be there yet.
And this is quite logical - the union only stores the value itself, but PHP obviously also needs to
store its type and some garbage collection information. The structure holding this information is
zval and you may have already have heard of it. For more information on why PHP needs it
I’d recommend to read an article by Sara Golemon. Anyways, this struct is defined as
The size of a struct is determined by the sum of the sizes of its components: The
16 bytes (as computed above), the
zend_uint is 4 bytes and the
zend_uchars are 1 byte each.
That’s a total of 22 bytes. Again due to memory alignment the real size will be 24 bytes though.
So if we store 100000 elements á 24 bytes that would be 2400000 in total, which is 2.29 MB. The gap is closing, but the real value is still more than six times larger.
The cycles collector (as of PHP 5.3)
PHP 5.3 introduced a new garbage collector for cyclic references. For this to work PHP has to
store some additional data. I don’t want to explain how the algorithm works here, you can read that
up on the linked page from the manual. Important for our size calculations is that PHP will wrap
zval into a
As you can see Zend only adds a union on top of it, which consists of two pointers. As you hopefully remember the size of a union is the size of its largest component: Both union components are pointers, thus both have a size of 8 bytes. So the size of the union is 8 bytes too.
If we add that on top of the 24 bytes we already have we get 32 bytes. Multiply that by the 100000 elements and we get a memory usage of 3.05 MB.
The Zend MM allocator
C unlike PHP does not manage memory for you. You need to keep track of your allocations yourself. For this purpose PHP uses a custom memory manager that is optimized specifically for its needs: The Zend Memory Manager. The Zend MM is based on Doug Lea’s malloc and adds some PHP specific optimizations and features (like memory limit, cleaning up after each request and stuff like that).
What is important for us here is that the MM adds an allocation header to every allocation done through it. It is defined as follows:
As you can see the definitions are cluttered with lots of compile option checks. So if one of those options is set the allocation header will be bigger and will be largest if you build PHP with heap protection, multi-threading, debug and MM cookies.
For this example though we will assume that all those options are disabled. In that case the only
thing left are the two
size_t has 8 bytes (on 64 bit), so the
allocation header has a total size of 16 bytes - and that header is added on every allocation.
So now we need to adjust our
zval size again. In reality it isn’t 32 bytes, but it’s 48, due to
that allocation header. Multiplied by our 100000 elements that’s 4.58 MB. The real value is 13.97
MB, so we already got approximately a third covered.
Until now we have only considered single values. But array structures in PHP also take up lots of
space: “Array” actually is a badly chosen term here. PHP arrays are hash tables / dictionaries in
reality. So how do hash tables work? Basically for every key a hash is generated and that hash is
used as an offset into a “real” C array. As the hashes can clash, all elements that have the same
hash are stored in a linked list. When accessing an element PHP first computes the hash, looks for
the right bucket and the traverses the link list, comparing the exact key, element by element. A
bucket is defined as follows (see
As you can see one needs to store loads of data to get the kind of abstract array data structure that PHP uses (PHP arrays are arrays, dictionaries and linked lists at the same time, that sure needs much info). The sizes of the individual components are 8 bytes for the unsigned long, 4 bytes for the unsigned int and 7 times 8 bytes for the pointers. That’s a total of 68. Add alignment and you get 72 bytes.
Buckets like zvals need to be allocated on the head, so we need to add the 16 bytes for the
allocation header again, giving us 88 bytes. Also we need to store pointers to those buckets in the
“real” C array (
Bucket **arBuckets;) I mentioned above, which adds another 8 bytes per element.
So all in all every bucket needs 96 bytes of storage.
So if we need a bucket for every value, that’s 96 bytes for the bucket and 48 bytes for the zval, which is 144 bytes in total. For 100000 elements that’s 14400000 bytes aka 13.73 MB.
Wait, there’s another 0.24 MB left!
Those last 0.24 MB are due to uninitialized buckets: The size of the real C array storing the buckets should ideally be approximately the same as the number of array elements stored. This way you have the least number of collisions (unless you want to waste lots of memory.) But PHP obviously can’t reallocate the whole array every time an element is added - that would be reeeally slow. Instead PHP always doubles the size of the internal bucket array if it hits the limit. So the size of the array is always a power of two.
In our case it is
2^17 = 131072. But we need only 100000 of those buckets, so we are leaving
31072 buckets unused. Those buckets will not be allocated (so we don’t need to spend the full 96
bytes), but the memory for the bucket pointer (the one stored in the internal bucket array) still
needs to be allocated. So we additionally use 8 bytes (a pointer) * 31072 elements. This is
248576 bytes or 0.23 MB. That matches the missing memory. (Sure, there are still a few bytes
missing, but I don’t really want to cover there. They are things like the hash table structure
itself, variables, etc.)
Mystery really solved.
What does this tell us?
PHP ain’t C. That’s all this should tell us. You can’t expect that a super dynamic language like PHP has the same highly efficient memory usage that C has. You just can’t.
But if you do want to save memory you could consider using an
SplFixedArray for large,
Have a look a this modified script:
It basically does the same thing, but if you run it, you’ll notice that it uses “only” 5600640 bytes. That’s 56 bytes per element and thus much less than the 144 bytes per element a normal array uses. This is because a fixed array doesn’t need the bucket structure: So it only requires one zval (48 bytes) and one pointer (8 bytes) for each element, giving us the observed 56 bytes.