mirror of git://gcc.gnu.org/git/gcc.git
560 lines
21 KiB
XML
560 lines
21 KiB
XML
<sect1 id="manual.ext.allocator.bitmap" xreflabel="bitmap_allocator">
|
|
<?dbhtml filename="bitmap_allocator.html"?>
|
|
|
|
<sect1info>
|
|
<keywordset>
|
|
<keyword>
|
|
ISO C++
|
|
</keyword>
|
|
<keyword>
|
|
allocator
|
|
</keyword>
|
|
</keywordset>
|
|
</sect1info>
|
|
|
|
<title>bitmap_allocator</title>
|
|
|
|
<para>
|
|
</para>
|
|
|
|
<sect2 id="allocator.bitmap.design">
|
|
<title>Design</title>
|
|
|
|
<para>
|
|
As this name suggests, this allocator uses a bit-map to keep track
|
|
of the used and unused memory locations for it's book-keeping
|
|
purposes.
|
|
</para>
|
|
<para>
|
|
This allocator will make use of 1 single bit to keep track of
|
|
whether it has been allocated or not. A bit 1 indicates free,
|
|
while 0 indicates allocated. This has been done so that you can
|
|
easily check a collection of bits for a free block. This kind of
|
|
Bitmapped strategy works best for single object allocations, and
|
|
with the STL type parameterized allocators, we do not need to
|
|
choose any size for the block which will be represented by a
|
|
single bit. This will be the size of the parameter around which
|
|
the allocator has been parameterized. Thus, close to optimal
|
|
performance will result. Hence, this should be used for node based
|
|
containers which call the allocate function with an argument of 1.
|
|
</para>
|
|
|
|
<para>
|
|
The bitmapped allocator's internal pool is exponentially growing.
|
|
Meaning that internally, the blocks acquired from the Free List
|
|
Store will double every time the bitmapped allocator runs out of
|
|
memory.
|
|
</para>
|
|
|
|
<para>
|
|
The macro <literal>__GTHREADS</literal> decides whether to use
|
|
Mutex Protection around every allocation/deallocation. The state
|
|
of the macro is picked up automatically from the gthr abstraction
|
|
layer.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
<sect2 id="allocator.bitmap.impl">
|
|
<title>Implementation</title>
|
|
|
|
<sect3 id="bitmap.impl.free_list_store" xreflabel="Free List Store">
|
|
<title>Free List Store</title>
|
|
|
|
<para>
|
|
The Free List Store (referred to as FLS for the remaining part of this
|
|
document) is the Global memory pool that is shared by all instances of
|
|
the bitmapped allocator instantiated for any type. This maintains a
|
|
sorted order of all free memory blocks given back to it by the
|
|
bitmapped allocator, and is also responsible for giving memory to the
|
|
bitmapped allocator when it asks for more.
|
|
</para>
|
|
<para>
|
|
Internally, there is a Free List threshold which indicates the
|
|
Maximum number of free lists that the FLS can hold internally
|
|
(cache). Currently, this value is set at 64. So, if there are
|
|
more than 64 free lists coming in, then some of them will be given
|
|
back to the OS using operator delete so that at any given time the
|
|
Free List's size does not exceed 64 entries. This is done because
|
|
a Binary Search is used to locate an entry in a free list when a
|
|
request for memory comes along. Thus, the run-time complexity of
|
|
the search would go up given an increasing size, for 64 entries
|
|
however, lg(64) == 6 comparisons are enough to locate the correct
|
|
free list if it exists.
|
|
</para>
|
|
<para>
|
|
Suppose the free list size has reached it's threshold, then the
|
|
largest block from among those in the list and the new block will
|
|
be selected and given back to the OS. This is done because it
|
|
reduces external fragmentation, and allows the OS to use the
|
|
larger blocks later in an orderly fashion, possibly merging them
|
|
later. Also, on some systems, large blocks are obtained via calls
|
|
to mmap, so giving them back to free system resources becomes most
|
|
important.
|
|
</para>
|
|
<para>
|
|
The function _S_should_i_give decides the policy that determines
|
|
whether the current block of memory should be given to the
|
|
allocator for the request that it has made. That's because we may
|
|
not always have exact fits for the memory size that the allocator
|
|
requests. We do this mainly to prevent external fragmentation at
|
|
the cost of a little internal fragmentation. Now, the value of
|
|
this internal fragmentation has to be decided by this function. I
|
|
can see 3 possibilities right now. Please add more as and when you
|
|
find better strategies.
|
|
</para>
|
|
|
|
<orderedlist>
|
|
<listitem><para>Equal size check. Return true only when the 2 blocks are of equal
|
|
size.</para></listitem>
|
|
<listitem><para>Difference Threshold: Return true only when the _block_size is
|
|
greater than or equal to the _required_size, and if the _BS is > _RS
|
|
by a difference of less than some THRESHOLD value, then return true,
|
|
else return false. </para></listitem>
|
|
<listitem><para>Percentage Threshold. Return true only when the _block_size is
|
|
greater than or equal to the _required_size, and if the _BS is > _RS
|
|
by a percentage of less than some THRESHOLD value, then return true,
|
|
else return false.</para></listitem>
|
|
</orderedlist>
|
|
|
|
<para>
|
|
Currently, (3) is being used with a value of 36% Maximum wastage per
|
|
Super Block.
|
|
</para>
|
|
</sect3>
|
|
|
|
<sect3 id="bitmap.impl.super_block" xreflabel="Super Block">
|
|
<title>Super Block</title>
|
|
|
|
<para>
|
|
A super block is the block of memory acquired from the FLS from
|
|
which the bitmap allocator carves out memory for single objects
|
|
and satisfies the user's requests. These super blocks come in
|
|
sizes that are powers of 2 and multiples of 32
|
|
(_Bits_Per_Block). Yes both at the same time! That's because the
|
|
next super block acquired will be 2 times the previous one, and
|
|
also all super blocks have to be multiples of the _Bits_Per_Block
|
|
value.
|
|
</para>
|
|
<para>
|
|
How does it interact with the free list store?
|
|
</para>
|
|
<para>
|
|
The super block is contained in the FLS, and the FLS is responsible for
|
|
getting / returning Super Bocks to and from the OS using operator new
|
|
as defined by the C++ standard.
|
|
</para>
|
|
</sect3>
|
|
|
|
<sect3 id="bitmap.impl.super_block_data" xreflabel="Super Block Data">
|
|
<title>Super Block Data Layout</title>
|
|
<para>
|
|
Each Super Block will be of some size that is a multiple of the
|
|
number of Bits Per Block. Typically, this value is chosen as
|
|
Bits_Per_Byte x sizeof(size_t). On an x86 system, this gives the
|
|
figure 8 x 4 = 32. Thus, each Super Block will be of size 32
|
|
x Some_Value. This Some_Value is sizeof(value_type). For now, let
|
|
it be called 'K'. Thus, finally, Super Block size is 32 x K bytes.
|
|
</para>
|
|
<para>
|
|
This value of 32 has been chosen because each size_t has 32-bits
|
|
and Maximum use of these can be made with such a figure.
|
|
</para>
|
|
<para>
|
|
Consider a block of size 64 ints. In memory, it would look like this:
|
|
(assume a 32-bit system where, size_t is a 32-bit entity).
|
|
</para>
|
|
|
|
<table frame='all'>
|
|
<title>Bitmap Allocator Memory Map</title>
|
|
<tgroup cols='5' align='left' colsep='1' rowsep='1'>
|
|
<colspec colname='c1'></colspec>
|
|
<colspec colname='c2'></colspec>
|
|
<colspec colname='c3'></colspec>
|
|
<colspec colname='c4'></colspec>
|
|
<colspec colname='c5'></colspec>
|
|
|
|
<tbody>
|
|
<row>
|
|
<entry>268</entry>
|
|
<entry>0</entry>
|
|
<entry>4294967295</entry>
|
|
<entry>4294967295</entry>
|
|
<entry>Data -> Space for 64 ints</entry>
|
|
</row>
|
|
</tbody>
|
|
</tgroup>
|
|
</table>
|
|
|
|
<para>
|
|
The first Column(268) represents the size of the Block in bytes as
|
|
seen by the Bitmap Allocator. Internally, a global free list is
|
|
used to keep track of the free blocks used and given back by the
|
|
bitmap allocator. It is this Free List Store that is responsible
|
|
for writing and managing this information. Actually the number of
|
|
bytes allocated in this case would be: 4 + 4 + (4x2) + (64x4) =
|
|
272 bytes, but the first 4 bytes are an addition by the Free List
|
|
Store, so the Bitmap Allocator sees only 268 bytes. These first 4
|
|
bytes about which the bitmapped allocator is not aware hold the
|
|
value 268.
|
|
</para>
|
|
|
|
<para>
|
|
What do the remaining values represent?</para>
|
|
<para>
|
|
The 2nd 4 in the expression is the sizeof(size_t) because the
|
|
Bitmapped Allocator maintains a used count for each Super Block,
|
|
which is initially set to 0 (as indicated in the diagram). This is
|
|
incremented every time a block is removed from this super block
|
|
(allocated), and decremented whenever it is given back. So, when
|
|
the used count falls to 0, the whole super block will be given
|
|
back to the Free List Store.
|
|
</para>
|
|
<para>
|
|
The value 4294967295 represents the integer corresponding to the bit
|
|
representation of all bits set: 11111111111111111111111111111111.
|
|
</para>
|
|
<para>
|
|
The 3rd 4x2 is size of the bitmap itself, which is the size of 32-bits
|
|
x 2,
|
|
which is 8-bytes, or 2 x sizeof(size_t).
|
|
</para>
|
|
</sect3>
|
|
|
|
<sect3 id="bitmap.impl.max_wasted" xreflabel="Max Wasted Percentage">
|
|
<title>Maximum Wasted Percentage</title>
|
|
|
|
<para>
|
|
This has nothing to do with the algorithm per-se,
|
|
only with some vales that must be chosen correctly to ensure that the
|
|
allocator performs well in a real word scenario, and maintains a good
|
|
balance between the memory consumption and the allocation/deallocation
|
|
speed.
|
|
</para>
|
|
<para>
|
|
The formula for calculating the maximum wastage as a percentage:
|
|
</para>
|
|
|
|
<para>
|
|
(32 x k + 1) / (2 x (32 x k + 1 + 32 x c)) x 100.
|
|
</para>
|
|
|
|
<para>
|
|
where k is the constant overhead per node (e.g., for list, it is
|
|
8 bytes, and for map it is 12 bytes) and c is the size of the
|
|
base type on which the map/list is instantiated. Thus, suppose the
|
|
type1 is int and type2 is double, they are related by the relation
|
|
sizeof(double) == 2*sizeof(int). Thus, all types must have this
|
|
double size relation for this formula to work properly.
|
|
</para>
|
|
<para>
|
|
Plugging-in: For List: k = 8 and c = 4 (int and double), we get:
|
|
33.376%
|
|
</para>
|
|
|
|
<para>
|
|
For map/multimap: k = 12, and c = 4 (int and double), we get: 37.524%
|
|
</para>
|
|
<para>
|
|
Thus, knowing these values, and based on the sizeof(value_type), we may
|
|
create a function that returns the Max_Wastage_Percentage for us to use.
|
|
</para>
|
|
|
|
</sect3>
|
|
|
|
<sect3 id="bitmap.impl.allocate" xreflabel="Allocate">
|
|
<title><function>allocate</function></title>
|
|
|
|
<para>
|
|
The allocate function is specialized for single object allocation
|
|
ONLY. Thus, ONLY if n == 1, will the bitmap_allocator's
|
|
specialized algorithm be used. Otherwise, the request is satisfied
|
|
directly by calling operator new.
|
|
</para>
|
|
<para>
|
|
Suppose n == 1, then the allocator does the following:
|
|
</para>
|
|
<orderedlist>
|
|
<listitem>
|
|
<para>
|
|
Checks to see whether a free block exists somewhere in a region
|
|
of memory close to the last satisfied request. If so, then that
|
|
block is marked as allocated in the bit map and given to the
|
|
user. If not, then (2) is executed.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Is there a free block anywhere after the current block right
|
|
up to the end of the memory that we have? If so, that block is
|
|
found, and the same procedure is applied as above, and
|
|
returned to the user. If not, then (3) is executed.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Is there any block in whatever region of memory that we own
|
|
free? This is done by checking
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
The use count for each super block, and if that fails then
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
The individual bit-maps for each super block.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
|
|
<para>
|
|
Note: Here we are never touching any of the memory that the
|
|
user will be given, and we are confining all memory accesses
|
|
to a small region of memory! This helps reduce cache
|
|
misses. If this succeeds then we apply the same procedure on
|
|
that bit-map as (1), and return that block of memory to the
|
|
user. However, if this process fails, then we resort to (4).
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
This process involves Refilling the internal exponentially
|
|
growing memory pool. The said effect is achieved by calling
|
|
_S_refill_pool which does the following:
|
|
</para>
|
|
<itemizedlist>
|
|
<listitem>
|
|
<para>
|
|
Gets more memory from the Global Free List of the Required
|
|
size.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Adjusts the size for the next call to itself.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Writes the appropriate headers in the bit-maps.
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
Sets the use count for that super-block just allocated to 0
|
|
(zero).
|
|
</para>
|
|
</listitem>
|
|
<listitem>
|
|
<para>
|
|
All of the above accounts to maintaining the basic invariant
|
|
for the allocator. If the invariant is maintained, we are
|
|
sure that all is well. Now, the same process is applied on
|
|
the newly acquired free blocks, which are dispatched
|
|
accordingly.
|
|
</para>
|
|
</listitem>
|
|
</itemizedlist>
|
|
</listitem>
|
|
</orderedlist>
|
|
|
|
<para>
|
|
Thus, you can clearly see that the allocate function is nothing but a
|
|
combination of the next-fit and first-fit algorithm optimized ONLY for
|
|
single object allocations.
|
|
</para>
|
|
|
|
</sect3>
|
|
|
|
<sect3 id="bitmap.impl.deallocate" xreflabel="Deallocate">
|
|
<title><function>deallocate</function></title>
|
|
<para>
|
|
The deallocate function again is specialized for single objects ONLY.
|
|
For all n belonging to > 1, the operator delete is called without
|
|
further ado, and the deallocate function returns.
|
|
</para>
|
|
<para>
|
|
However for n == 1, a series of steps are performed:
|
|
</para>
|
|
|
|
<orderedlist>
|
|
<listitem><para>
|
|
We first need to locate that super-block which holds the memory
|
|
location given to us by the user. For that purpose, we maintain
|
|
a static variable _S_last_dealloc_index, which holds the index
|
|
into the vector of block pairs which indicates the index of the
|
|
last super-block from which memory was freed. We use this
|
|
strategy in the hope that the user will deallocate memory in a
|
|
region close to what he/she deallocated the last time around. If
|
|
the check for belongs_to succeeds, then we determine the bit-map
|
|
for the given pointer, and locate the index into that bit-map,
|
|
and mark that bit as free by setting it.
|
|
</para></listitem>
|
|
<listitem><para>
|
|
If the _S_last_dealloc_index does not point to the memory block
|
|
that we're looking for, then we do a linear search on the block
|
|
stored in the vector of Block Pairs. This vector in code is
|
|
called _S_mem_blocks. When the corresponding super-block is
|
|
found, we apply the same procedure as we did for (1) to mark the
|
|
block as free in the bit-map.
|
|
</para></listitem>
|
|
</orderedlist>
|
|
|
|
<para>
|
|
Now, whenever a block is freed, the use count of that particular
|
|
super block goes down by 1. When this use count hits 0, we remove
|
|
that super block from the list of all valid super blocks stored in
|
|
the vector. While doing this, we also make sure that the basic
|
|
invariant is maintained by making sure that _S_last_request and
|
|
_S_last_dealloc_index point to valid locations within the vector.
|
|
</para>
|
|
</sect3>
|
|
|
|
<sect3 id="bitmap.impl.questions" xreflabel="Questions">
|
|
<title>Questions</title>
|
|
|
|
<sect4 id="bitmap.impl.question.1" xreflabel="Question 1">
|
|
<title>1</title>
|
|
<para>
|
|
Q1) The "Data Layout" section is
|
|
cryptic. I have no idea of what you are trying to say. Layout of what?
|
|
The free-list? Each bitmap? The Super Block?
|
|
</para>
|
|
<para>
|
|
The layout of a Super Block of a given
|
|
size. In the example, a super block of size 32 x 1 is taken. The
|
|
general formula for calculating the size of a super block is
|
|
32 x sizeof(value_type) x 2^n, where n ranges from 0 to 32 for 32-bit
|
|
systems.
|
|
</para>
|
|
</sect4>
|
|
|
|
<sect4 id="bitmap.impl.question.2" xreflabel="Question 2">
|
|
<title>2</title>
|
|
<para>
|
|
And since I just mentioned the
|
|
term `each bitmap', what in the world is meant by it? What does each
|
|
bitmap manage? How does it relate to the super block? Is the Super
|
|
Block a bitmap as well?
|
|
</para>
|
|
<para>
|
|
Each bitmap is part of a Super Block which is made up of 3 parts
|
|
as I have mentioned earlier. Re-iterating, 1. The use count,
|
|
2. The bit-map for that Super Block. 3. The actual memory that
|
|
will be eventually given to the user. Each bitmap is a multiple
|
|
of 32 in size. If there are 32 x (2^3) blocks of single objects
|
|
to be given, there will be '32 x (2^3)' bits present. Each 32
|
|
bits managing the allocated / free status for 32 blocks. Since
|
|
each size_t contains 32-bits, one size_t can manage up to 32
|
|
blocks' status. Each bit-map is made up of a number of size_t,
|
|
whose exact number for a super-block of a given size I have just
|
|
mentioned.
|
|
</para>
|
|
</sect4>
|
|
|
|
<sect4 id="bitmap.impl.question.3" xreflabel="Question 3">
|
|
<title>3</title>
|
|
<para>
|
|
How do the allocate and deallocate functions work in regard to
|
|
bitmaps?
|
|
</para>
|
|
<para>
|
|
The allocate and deallocate functions manipulate the bitmaps and
|
|
have nothing to do with the memory that is given to the user. As
|
|
I have earlier mentioned, a 1 in the bitmap's bit field
|
|
indicates free, while a 0 indicates allocated. This lets us
|
|
check 32 bits at a time to check whether there is at lease one
|
|
free block in those 32 blocks by testing for equality with
|
|
(0). Now, the allocate function will given a memory block find
|
|
the corresponding bit in the bitmap, and will reset it (i.e.,
|
|
make it re-set (0)). And when the deallocate function is called,
|
|
it will again set that bit after locating it to indicate that
|
|
that particular block corresponding to this bit in the bit-map
|
|
is not being used by anyone, and may be used to satisfy future
|
|
requests.
|
|
</para>
|
|
<para>
|
|
e.g.: Consider a bit-map of 64-bits as represented below:
|
|
1111111111111111111111111111111111111111111111111111111111111111
|
|
</para>
|
|
|
|
<para>
|
|
Now, when the first request for allocation of a single object
|
|
comes along, the first block in address order is returned. And
|
|
since the bit-maps in the reverse order to that of the address
|
|
order, the last bit (LSB if the bit-map is considered as a
|
|
binary word of 64-bits) is re-set to 0.
|
|
</para>
|
|
|
|
<para>
|
|
The bit-map now looks like this:
|
|
1111111111111111111111111111111111111111111111111111111111111110
|
|
</para>
|
|
</sect4>
|
|
</sect3>
|
|
|
|
<sect3 id="bitmap.impl.locality" xreflabel="Locality">
|
|
<title>Locality</title>
|
|
<para>
|
|
Another issue would be whether to keep the all bitmaps in a
|
|
separate area in memory, or to keep them near the actual blocks
|
|
that will be given out or allocated for the client. After some
|
|
testing, I've decided to keep these bitmaps close to the actual
|
|
blocks. This will help in 2 ways.
|
|
</para>
|
|
|
|
<orderedlist>
|
|
<listitem><para>Constant time access for the bitmap themselves, since no kind of
|
|
look up will be needed to find the correct bitmap list or it's
|
|
equivalent.</para></listitem>
|
|
<listitem><para>And also this would preserve the cache as far as possible.</para></listitem>
|
|
</orderedlist>
|
|
|
|
<para>
|
|
So in effect, this kind of an allocator might prove beneficial from a
|
|
purely cache point of view. But this allocator has been made to try and
|
|
roll out the defects of the node_allocator, wherein the nodes get
|
|
skewed about in memory, if they are not returned in the exact reverse
|
|
order or in the same order in which they were allocated. Also, the
|
|
new_allocator's book keeping overhead is too much for small objects and
|
|
single object allocations, though it preserves the locality of blocks
|
|
very well when they are returned back to the allocator.
|
|
</para>
|
|
</sect3>
|
|
|
|
<sect3 id="bitmap.impl.grow_policy" xreflabel="Grow Policy">
|
|
<title>Overhead and Grow Policy</title>
|
|
<para>
|
|
Expected overhead per block would be 1 bit in memory. Also, once
|
|
the address of the free list has been found, the cost for
|
|
allocation/deallocation would be negligible, and is supposed to be
|
|
constant time. For these very reasons, it is very important to
|
|
minimize the linear time costs, which include finding a free list
|
|
with a free block while allocating, and finding the corresponding
|
|
free list for a block while deallocating. Therefore, I have
|
|
decided that the growth of the internal pool for this allocator
|
|
will be exponential as compared to linear for
|
|
node_allocator. There, linear time works well, because we are
|
|
mainly concerned with speed of allocation/deallocation and memory
|
|
consumption, whereas here, the allocation/deallocation part does
|
|
have some linear/logarithmic complexity components in it. Thus, to
|
|
try and minimize them would be a good thing to do at the cost of a
|
|
little bit of memory.
|
|
</para>
|
|
|
|
<para>
|
|
Another thing to be noted is the pool size will double every time
|
|
the internal pool gets exhausted, and all the free blocks have
|
|
been given away. The initial size of the pool would be
|
|
sizeof(size_t) x 8 which is the number of bits in an integer,
|
|
which can fit exactly in a CPU register. Hence, the term given is
|
|
exponential growth of the internal pool.
|
|
</para>
|
|
</sect3>
|
|
|
|
</sect2>
|
|
|
|
</sect1>
|