Changeset 0dffe91

Timestamp:

Nov 18, 2024, 11:49:23 AM (3 hours ago)

Author:

Michael Brooks <mlbrooks@…>

Branches:

master

Parents:

ea10f64

Message:

Thesis, string: add missing figure

Location:

doc/theses/mike_brooks_MMath

Files:

• pictures/memmgr-basic.pdf (added)
• pictures/memmgr-basic.vsdx (added)
• string.tex (modified) (1 diff)

doc/theses/mike_brooks_MMath/string.tex

@@ -161 +161 @@
 This cycle of frequent cheap allocations, interspersed with infrequent expensive compactions, has obvious similarities to a general-purpose memory manager based on garbage collection (GC).  A few differences are noteworthy.  First, in a general purpose manager, the objects of allocation contain pointers to other such objects, making the transitive reachability of these objects be a critical property.  Here, the allocations are of buffers of text, never pointers, so one allocation never keeps another one alive.  Second, in a general purpose manager, where the handle that keeps an allocation alive is the same as the program's general-purpose inter-object reference, an extremely lean representation of this reference is required.  Here, a fatter representation is acceptable because [why??].
 
-
-Figure [memmgr-basix.vsdx] shows the representation.  A heap header, with its text buffer, defines a sharing context.  Often, one global sharing context is appropriate for an entire program; exceptions are discussed in [xref TBD].  Strings are handles into the buffer.  They are members of a linked list whose order matches the order of their buffer fragments (exactly, where there is no overlapping, and approximately, where there is).  The header maintains a next-allocation pointer (alloc, in the figure) after the last live allocation of the buffer.  No external references into the buffer are allowed and the management procedure relocates the text allocations as needed.  The string handles contain explicit length fields, null termination characters are not used and all string text is kept in contiguous storage.  When strings (the inter-linked handles) are allocated on the program's call stack, a sustained period with no use of the program's dynamic memory allocator can ensue, during which the program nonetheless creates strings, destroys them, and runs length-increasing modifications on existing ones.  
+\begin{figure}
+\includegraphics{memmgr-basic}
+\caption{String memory-management data structures}
+\label{f:memmgr-basic}
+\end{figure}
+
+\VRef[Figure]{f:memmgr-basic} shows the representation.  A heap header, with its text buffer, defines a sharing context.  Often, one global sharing context is appropriate for an entire program; exceptions are discussed in [xref TBD].  Strings are handles into the buffer.  They are members of a linked list whose order matches the order of their buffer fragments (exactly, where there is no overlapping, and approximately, where there is).  The header maintains a next-allocation pointer (alloc, in the figure) after the last live allocation of the buffer.  No external references into the buffer are allowed and the management procedure relocates the text allocations as needed.  The string handles contain explicit length fields, null termination characters are not used and all string text is kept in contiguous storage.  When strings (the inter-linked handles) are allocated on the program's call stack, a sustained period with no use of the program's dynamic memory allocator can ensue, during which the program nonetheless creates strings, destroys them, and runs length-increasing modifications on existing ones.  
 
 Compaction happens when the heap fills.  It is the first of two uses of the linked list.  The list allows discovering all live string handles, and from them, the ranges of the character buffer that are in use.  With these ranges known, their total character count gives the amount of space in use.  When that amount is small, compared with the current buffer size, an in-place compaction occurs, which entails copying the in-use ranges, to be adjacent, at the font of the buffer.  When the in-use amount is large, a larger buffer is allocated (using the program's general-purpose dynamic allocator), the in-use strings are copied to be adjacent at the front of it, and the original buffer is freed back to the program's general allocator.  Either way, navigating the links between the handles provides the pointers into the buffer, first read, to find the source fragment, then written with the location of the resultant fragment.  This linkage across the structure is unaffected during a compaction; only the pointers from the handles to the buffer are modified.  This modification, along with the grooming/provisioning of the text storage resource that it represents, is an example, in the language of [xref RAII limitations] of the string module providing a ``stay good'' service.