FAT-Allocator/docs/FutureTasks/docs.org

#+options: num:0

* Future work
This documents is decision making to highlight
potential paths to take for this PhD.
We will initially talk about the current expirement
which is a FAT pointer based memory allocator
and will then expand into 2 potential paths:

- Cheri RISCV to prevent using the TLB.
- Allocator evaluation based on stripping instruction
  calls for larger allocators like Jemalloc\cite{noauthor_jemalloc_nodate}.


** 1. Current expirement: FAT pointer based range addresses
#+NAME:   fig:FPBRA
#+CAPTION: FAT pointer implementation with Huge pages against a standard malloc allocator.
[[./HighOverviewArchitecture.drawio.png]]

The objective of this expirement was to ensure we can use the CHERI bounds as
tracking mechanism of allocations instead of using multiple TLB entries. Using
this approach we can use a single Huge page\cite{navarro_practical_nodate} entry with bounds to ensure that
the bounds (Which is the top and base address) can be extracted from the
pointer using the Cheri compressed bounds\cite{woodruff_cheri_2019} mechanism. We implemented a simple
allocator which uses this technique with a basic malloc and free.

*** Objectives 
- How does the utilization of bounds for tracking memory
  allocations, in addition to security purposes, affect
  the run times and Translation Lookaside Buffer (TLB)
  miss rates in modern computing systems ?
  
- How does the implementation of bounds for seeking through
  physically contiguous memory influence the complexity and
  efficiency of standard memory allocators, particularly
  those with advanced features such as transparent huge pages,
  and what are the implications for system performance in terms
  of execution speed, memory access latency, and resource
  utilization?
  
*** Hardware
- ARM morello\cite{noauthor_department_nodate} (Huge page size 1GB used)

*** Evaluation
    We conducted tests of the FAT Pointer-based range addresses against Jemalloc,
    the default memory allocator for CHERIBSD, to assess the
    performance improvements enabled by a CHERI-based huge page-aware
    alocator. Specifically, we evaluated the reduction in TLB misses
    and its impact on overall performance metrics, such as wall clock runtime.
    To comprehensively analyze the proposed allocator,
we categorized benchmarks into two classes which are micro and
macro benchmarks. Micro benchmarks comprise smaller C programs
designed to target specific allocator patterns, enabling us
to evaluate detailed aspects of the allocator’s behavior.
Macro benchmarks, on the other hand, encompass larger,
realworld C programs, allowing us to assess the allocator’s
performance in more practical, real-world scenarios.

*** limitation
- Using Huge page still requires a TLB entry which could be mitigated
  (Refer to the FPGA work).
- ARMv8 only supports using to virtual addresses so it's required to
  bypass the TLB for address translation.

  
** 2. Cheri RISCV to prevent using the TLB

#+attr_latex: :width 200px
#+CAPTION: FAT pointer implementation with RISCV CHERI Toooba to strip the requirement of requiring a TLB.  
#+NAME: fig:RFPBRA
[[./MainOverview.png]]

In the current ARM Morello setup, address
translations rely on the TLB. The future approach
on RISC-V Tooba\cite{noauthor_ctsrd-cheritoooba_nodate} involves storing the offset directly within the pointer.
This is possible due to CHERI’s capability model, which supports
fine-grained memory protection and can encode bounds within pointers.
Utilizing Bounds in CHERI for Block-Based Allocation:
CHERI capabilities allow pointers to carry metadata about memory bounds,
providing hardware-enforced memory safety. By encoding the offset
and bounds within the pointer, the system can directly access memory
without needing intermediate translations via the TLB. This enables the
implementation of a block-based allocator that can efficiently manage memory
allocations and deallocations within defined bounds. Bypassing the TLB in RISC-V Tooba.

*** Hardware modifications
The Bluespec design of the RISC-V processor will be modified to allow
certain memory operations to bypass the TLB. This means that when a pointer
with encoded offset and bounds is used, the system can directly compute the
physical address from the capability information. This modification reduces the
dependency on the TLB, decreasing latency.
and improving performance, especially for frequent memory operations.

** 3. Allocator evaluation based on stripping instruction calls for larger allocators

#+NAME: fig:MEMALLOC
#+CAPTION: Deprecating the use of THP with CHERI bound aware embedded mmap. 
[[./memory_allocator.drawio.png]]

*** Box 1 (Transparent huge pages)
#+BEGIN_COMMENT
The diagram above mentions 3 particular implementations. The first box which is the
standard THP(Transparent huge pages) utilised by modern allocators. THP initially
emphasises on doing smalled allocations and as the number of allocations grows
uses a technique which groups all smaller allocations together and when done
converts them into a large page of size 4mb in allocators such as jemalloc.
#+END_COMMENT

The diagram [[fig:MEMALLOC]] highlights three specific implementations, the
first of which is the standard Transparent Huge Pages (THP)
mechanism employed by modern memory allocators. THP initially
focuses on handling smaller memory allocations. As the volume
of allocations increases, it employs a strategy that consolidates
these smaller allocations into contiguous memory regions.
Once aggregated, these regions are subsequently converted
into larger memory pages, typically of size 4MB, as seen
in allocators like jemalloc. This approach optimizes memory
management by reducing fragmentation and improving performance
through the use of larger page sizes.

#+BEGIN_COMMENT
This approach does incur addtional operations such as grouping smaller allocations
chaging the TLB entries (Adding more oppurtunity for TLB misses). Only once the
huge page is created the TLB misses are reduced.
#+END_COMMENT
This approach, however, introduces additional
overhead, including the operations required to consolidate
smaller allocations and modify Translation Lookaside Buffer (TLB)
entries. These modifications can initially increase the
likelihood of TLB misses, as the process of grouping and
reorganizing memory allocations temporarily disrupts the
efficiency of TLB utilization. It is only after the
successful creation of the huge page that the benefits
materialize, leading to a reduction in TLB misses due
to the improved alignment of memory access patterns with
the larger page size.

*** Box 2 (Our current implementation)
#+BEGIN_COMMENT
Box 2 which refers to our current implementation always pre-allocates huge pages
and untilises CHERI bounds to track each allocation inside the huge page. Allowing
a single entry with the combination of bounds to provide block based behavoir in
physically contigous memory while ensuring a pointer can only access a regoin
within it's defined bounds.
#+END_COMMENT

Box 2 outlines the current implementation, which involves the
pre-allocation of huge pages and leverages CHERI
(Capability Hardware Enhanced RISC Instructions) bounds
to meticulously track each allocation within these pages.
This approach enables a single TLB entry, combined with
the precise bounds defined by CHERI capabilities, to
facilitate block-based memory management within physically
contiguous regions. By enforcing strict bounds on pointers,
the system ensures that each pointer can only access memory
within its explicitly defined region.

#+BEGIN_COMMENT
Another aspect to note is that the bounds can be of a dynamic size when defined. This is
in contrast to defining multiple page entries which need to be fixed sizes which means
they always incur multiple entries. In the current approach when the huge page size is
hit a new one is created. The limitaton of this is appraoch being we are limited to the
huge page set by the processor implementation (In our case the CHERI ARM v8.1).
#+END_COMMENT

Another critical aspect of this implementation is the
ability to define bounds of dynamic sizes, which stands
in contrast to traditional approaches that rely on
fixed-size page entries. Fixed-size entries inherently
require multiple TLB entries, regardless of the actual
memory usage, leading to inefficiencies. In the current
approach, when the allocated memory within a huge page
reaches its capacity, a new huge page is allocated.
However, a notable limitation of this method is its
dependence on the maximum huge page size supported by
the underlying processor architecture. In this case,
the system is constrained by the huge page size defined
by the CHERI-enhanced ARM v8.1\cite{noauthor_arm_nodate} architecture. While this
approach offers flexibility in memory allocation and
reduces the need for multiple TLB entries, it is
ultimately bounded by the hardware's architectural specifications.

*** Box 3 (RISC-V implementation)
#+BEGIN_COMMENT
The 3rd box specifies an alternate appraoch by not using huge pages and required
memory is not required to be physically contigous. In this approach the pointer
stores all the metadata to the translation from virtual to physical addresses.
#+END_COMMENT

The third approach, as outlined in Box 3, deviates from the
use of huge pages and does not require memory to be
physically contiguous. In this model, each pointer
is designed to store comprehensive metadata at the pointer necessary
for the translation from virtual to physical addresses.
This metadata enables the system to manage memory allocations
without the constraints of physical contiguity, thereby
offering greater flexibility in memory utilization.
By embedding translation information directly within the
pointers, this approach eliminates the need for large,
contiguous memory regions and allows for more granular
and dynamic memory management.

*** Building up from the work of Box 2 and Box 3 (Side effects we can strip away)
#+BEGIN_COMMENT
Box 2 and 3 from a high overview there is only minor difference which can be noted
which is 1 uses huge pages and other does not. Both approaches can strip down the
number intructions needed in modern allocators (Stripping away the need transitioning
from smaller to larger pages). This document is yet to give an exact breakdown.
#+END_COMMENT

From a high-level perspective, the primary distinction between
Box 2 and Box 3 lies in the use of huge pages in the former
and their absence in the latter. Both approaches share
the common advantage of reducing the number of instructions
required in modern memory allocators by eliminating the
need for transitioning between smaller and larger pages.
This simplification streamlines memory management processes.
However, this document has not yet provided a detailed breakdown
or quantitative analysis of the specific performance implications
or trade-offs.

#+BEGIN_COMMENT
As seen to the right of the diagram is a sample snippet of TC malloc from the paper
(Beyond malloc efficiency to fleet allocators). This whole span function would not
be required in our approach. The other benefit being easier get the approach by
getting mmap embedded inside the allocator.
#+END_COMMENT

As illustrated to the right of the diagram, a sample snippet of
TC malloc from the paper Beyond malloc Efficiency to Fleet
Efficiency is provided. In the proposed approach, the entire
span function, which is essential in TC malloc, would become
unnecessary. Additionally, the approach offers the advantage
of simplifying memory management by integrating mmap directly
within the allocator. This integration eliminates the need for
separate mechanisms to handle memory mapping.

*** Evaluation:
#+BEGIN_COMMENT
- Amount of instructions that can be stripped away from the page aware
  memory allocator.
- Comparing memory allocator with wall clock run time with the modified mmap and without the modified mmap.
- CHERI purecap does incur additional instruction such as bound checks. Does this approach as a whole
  reduce the number of instructions as whole (Comparing CHERIpurecap instructions with memory allocator
  emitted vs regular ARMv8 clang program with the same allocator). 
#+END_COMMENT

- The number of instructions that can be eliminated from a
  page-aware memory allocator by adopting the proposed approach.

- A comparative analysis of the memory allocator's performance
  using wall-clock runtime measurements, both with and without
  the modified mmap implementation.

- While CHERI Purecap introduces additional instructions, such
  as bounds checks, the overall approach aims to determine
  whether it reduces the total number of instructions when
  compared to a traditional ARMv8 Clang program using the
  same allocator. This involves evaluating the trade-offs
  between the overhead of CHERI-specific instructions and
  the potential reductions in allocator-emitted instructions.

** Current tasks on hand
1. To port over all chapters written to a
   EuroSys Conferenece document and send over the
   to the supervisory team for a review.
   - This will then get send over to Jeremy
     for an external review.
   - Expands to a talk in the University
     of Glasgow seminar.
   - Concurrent suggestions of the Eurosys
     submission till may.
2. Work on allocator evaluation based on stripping instruction calls for
   larger allocators (Expirement agreed in the meeting). 
3. Write to Cambridge a proposal for using the RISC-V expirement (2 weeks).
    
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