Understanding The Memcached Source Code — Slab II

Photo by Audrey Fretz on Unsplash

slab allocator (I, II — this article) is the core module of the cache system, which largely determines how efficient the bottleneck resource, memory, can be utilized. The other 3 parts, namely,

LRU algorithm (not complete) for entry expiration; and an

event driven model (not complete) based on libevent; and the

consistent harsh (not complete) for data distribution,

are built around it.

This time we continue examining how slabs memory is allocated.

Firstly we look at the two arguments for slabs_init, which were passed over in the previous article. The first one issettings.maxbytes, it limits the overall memory that can be used by the memcached instance. In slabs_init, the value of settings.maxbytes is assigned to the global variable mem_limit which will be used very soon.

The other argument is preallocate. It determines whether to preallocate slab for each slab class. This argument is toggled with L command line argument.

Next we look at the method for slabs memory allocation itself.

New slab

More specific, this method allocate one 1M sized slab for the slab class indicated by the parameter id.

1) slabclass[id] is one of the slab class, the initialization of which is discussed in last article.

2) settings.slab_reassign determines whether to enlist a rebalancing mechanism, which recycles the unused slabs and redistributes them across slab classes. This requires that slabs contained in all slab classes be of the same size, hence this setting also decides whether to use unanimous (i.e., settings.item_size_max, or 1M as mentioned before) or heterogeneous (i.e., p->size * p->perslab) slabs. Besides the its associated command line argument "slab_reassign", the value can be controlled by another argument "modern". For the positivity the name “modern” implies, 1M will be used throughout the text.

N.b. *, rebalancing mechanism will be discussed later when we have a better understanding of the LRU module.

3) Check if the memory usage will exceeds the upper limit.

4) grow_slab_list checks if we need to increase slabclass_t.slab_list, if so, grows it.

5) memory_allocate allocates the actual memory for the slab. As discussed, here the value of len is 1M.

6) split_slab_page_into_freelist initializes (frees) the newly allocated slab preparing for objects storing. This method will be discussed in the next section.

7) Add the newly allocated slab to the slabclass_t.slab_list.

What has happened so far can be summarized with the following figure, (we assume do_slabs_newslab(n) is called two times)

Now we look inside the 1M slab in step 6).

split_slab_page_into_freelist

This method goes through all the item chunks (in the size of slabclass_t.size) within a slab. And for each of them, the method initializes its meta data by calling do_slabs_free. Another way to interpret this process is “split a slab into item free list”. As you might have already figured out, this “free list” will be used by item allocation in future.

do_slabs_free

This method works on item meta data that is populated in the beginning of an item memory chunk.

1) Initialize some fields. item is another core data structure, we will come back to item data structure later.

2) Add the item to the front of the linked list (a.k.a., free list). And update the list head, slabclass_t.slots.

3) Update the available (free list) slot count, slabclass_t.sl_curr; and updates the slabclass_t.requested for statistic. Note that here we are not actually releasing an item, so the passed size is 0.

Slab preallocate

Next we look at how do_slabs_newslab is used. One place it gets called is from the discussed slabs_init when preallocate is set to true,

This method simply goes through the slabclass starting from the POWER_SMALLEST, i.e., 1st entry, and allocate one slab for each of them. Note that the 0th is a special slab class used by mentioned rebalancing mechanism.

References

Same to the previous one.


Originally published at holmeshe.me.