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How big are PHP arrays (and values) really? (Hint: BIG!)

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:

$startMemory = memory_get_usage();
$array = range(1, 100000);
echo memory_get_usage() - $startMemory, ' bytes';

How much would you expect it to be? Simple, one integer is 8 bytes (on a 64 bit unix machine and using the 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?

Summary

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 :)

The zvalue_value union

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):

typedef union _zvalue_value {
    long lval;                // For integers and booleans
    double dval;              // For floats (doubles)
    struct {                  // For strings
        char *val;            //     consisting of the string itself
        int len;              //     and its length
    } str;
    HashTable *ht;            // For arrays (hash tables)
    zend_object_value obj;    // For objects
} zvalue_value;

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 zvalue_value->lval 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 union 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.

The zval struct

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 called a 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 follows:

struct _zval_struct {
    zvalue_value value;     // The value
    zend_uint refcount__gc; // The number of references to this value (for GC)
    zend_uchar type;        // The type
    zend_uchar is_ref__gc;  // Whether this value is a reference (&)
};

The size of a struct is determined by the sum of the sizes of its components: The zvalue_value is 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 every zval into a zval_gc_info:

typedef struct _zval_gc_info {
    zval z;
    union {
        gc_root_buffer       *buffered;
        struct _zval_gc_info *next;
    } u;
} zval_gc_info;

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:

typedef struct _zend_mm_block {
    zend_mm_block_info info;
#if ZEND_DEBUG
    unsigned int magic;
# ifdef ZTS
    THREAD_T thread_id;
# endif
    zend_mm_debug_info debug;
#elif ZEND_MM_HEAP_PROTECTION
    zend_mm_debug_info debug;
#endif
} zend_mm_block;

typedef struct _zend_mm_block_info {
#if ZEND_MM_COOKIES
    size_t _cookie;
#endif
    size_t _size; // size of the allocation
    size_t _prev; // previous block (not sure what exactly this is)
} zend_mm_block_info;

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_ts _size and _prev. A 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.

Buckets

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 zend_hash.h#54):

typedef struct bucket {
    ulong h;                  // The hash (or for int keys the key)
    uint nKeyLength;          // The length of the key (for string keys)
    void *pData;              // The actual data
    void *pDataPtr;           // ??? What's this ???
    struct bucket *pListNext; // PHP arrays are ordered. This gives the next element in that order
    struct bucket *pListLast; // and this gives the previous element
    struct bucket *pNext;     // The next element in this (doubly) linked list
    struct bucket *pLast;     // The previous element in this (doubly) linked list
    const char *arKey;        // The key (for string keys)
} Bucket;

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.

Mystery solved.

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, static arrays.

Have a look a this modified script:

$startMemory = memory_get_usage();
$array = new SplFixedArray(100000);
for ($i = 0; $i < 100000; ++$i) {
    $array[$i] = $i;
}
echo memory_get_usage() - $startMemory, ' bytes';

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.