private void AllocateNextSegment(int required, bool allowGrowth)
{
Debug.Assert(required > 0);
// Grow by doubling segment size until we get to 1 MB, then just use 1 MB segments
// otherwise a document with 17 MB will waste 15 MB and require very big allocations
var powerOfTwoRequestSize = Bits.NextPowerOf2(required);
var segmentSize = Math.Max(powerOfTwoRequestSize, _head.Allocation.SizeInBytes * 2);
if (powerOfTwoRequestSize < segmentSize ||
segmentSize > ArenaMemoryAllocator.MaxArenaSize)
{
// don't grow _too_ much.
segmentSize = powerOfTwoRequestSize;
}
const int oneMb = 1024 * 1024;
if (segmentSize > oneMb && required <= oneMb)
{
segmentSize = oneMb;
}
// We can sometimes ask the context to grow the allocation size;
// it may do so at its own discretion; if this happens, then we
// are good to go.
if (allowGrowth && _context.GrowAllocation(_head.Allocation, segmentSize))
{
return;
}
// Can't change _head because there may be copies of the current
// instance of UnmanagedWriteBuffer going around. Thus, we simply
// mutate it to ensure all copies have the same allocations.
var allocation = _context.GetMemory(segmentSize);
// Copy the head
Segment previousHead = _head.ShallowCopy();
// Reset the head (this change happens in all instances at the
// same time, albeit not atomically).
_head.Previous = previousHead;
_head.DeallocationPendingPrevious = previousHead;
_head.Allocation = allocation;
_head.Address = allocation.Address;
_head.Used = 0;
_head.AccumulatedSizeInBytes = previousHead.AccumulatedSizeInBytes;
}