==================
QATzip Compression
==================
In scenarios with limited network bandwidth, the ``QATzip`` solution can help
users save a lot of host CPU resources by accelerating compression and
decompression through the Intel QuickAssist Technology(``QAT``) hardware.


The following test was conducted using 8 multifd channels and 10Gbps network
bandwidth. The results show that, compared to zstd, ``QATzip`` significantly
saves CPU resources on the sender and reduces migration time. Compared to the
uncompressed solution, ``QATzip`` greatly improves the dirty page processing
capability, indicated by the Pages per Second metric, and also reduces the
total migration time.

::

   VM Configuration: 16 vCPU and 64G memory
   VM Workload: all vCPUs are idle and 54G memory is filled with Silesia data.
   QAT Devices: 4
   |-----------|--------|---------|----------|----------|------|------|
   |8 Channels |Total   |down     |throughput|pages per | send | recv |
   |           |time(ms)|time(ms) |(mbps)    |second    | cpu %| cpu% |
   |-----------|--------|---------|----------|----------|------|------|
   |qatzip     |   16630|       28|     10467|   2940235|   160|   360|
   |-----------|--------|---------|----------|----------|------|------|
   |zstd       |   20165|       24|      8579|   2391465|   810|   340|
   |-----------|--------|---------|----------|----------|------|------|
   |none       |   46063|       40|     10848|    330240|    45|    85|
   |-----------|--------|---------|----------|----------|------|------|


QATzip Compression Framework
============================

``QATzip`` is a user space library which builds on top of the Intel QuickAssist
Technology to provide extended accelerated compression and decompression
services.

For more ``QATzip`` introduction, please refer to `QATzip Introduction
<https://github.com/intel/QATzip?tab=readme-ov-file#introductionl>`_

::

  +----------------+
  | MultiFd Thread |
  +-------+--------+
          |
          | compress/decompress
  +-------+--------+
  | QATzip library |
  +-------+--------+
          |
  +-------+--------+
  |  QAT library   |
  +-------+--------+
          |         user space
  --------+---------------------
          |         kernel space
   +------+-------+
   |  QAT  Driver |
   +------+-------+
          |
   +------+-------+
   | QAT Devices  |
   +--------------+


QATzip Installation
-------------------

The ``QATzip`` installation package has been integrated into some Linux
distributions and can be installed directly. For example, the Ubuntu Server
24.04 LTS system can be installed using below command

.. code-block:: shell

   #apt search qatzip
   libqatzip-dev/noble 1.2.0-0ubuntu3 amd64
     Intel QuickAssist user space library development files

   libqatzip3/noble 1.2.0-0ubuntu3 amd64
     Intel QuickAssist user space library

   qatzip/noble,now 1.2.0-0ubuntu3 amd64 [installed]
     Compression user-space tool for Intel QuickAssist Technology

   #sudo apt install libqatzip-dev libqatzip3 qatzip

If your system does not support the ``QATzip`` installation package, you can
use the source code to build and install, please refer to `QATzip source code installation
<https://github.com/intel/QATzip?tab=readme-ov-file#build-intel-quickassist-technology-driver>`_

QAT Hardware Deployment
-----------------------

``QAT`` supports physical functions(PFs) and virtual functions(VFs) for
deployment, and users can configure ``QAT`` resources for migration according
to actual needs. For more details about ``QAT`` deployment, please refer to
`Intel QuickAssist Technology Documentation
<https://intel.github.io/quickassist/index.html>`_

For more ``QAT`` hardware introduction, please refer to `intel-quick-assist-technology-overview
<https://www.intel.com/content/www/us/en/architecture-and-technology/intel-quick-assist-technology-overview.html>`_

How To Use QATzip Compression
=============================

1 - Install ``QATzip`` library

2 - Build ``QEMU`` with ``--enable-qatzip`` parameter

  E.g. configure --target-list=x86_64-softmmu --enable-kvm ``--enable-qatzip``

3 - Set ``migrate_set_parameter multifd-compression qatzip``

4 - Set ``migrate_set_parameter multifd-qatzip-level comp_level``, the default
comp_level value is 1, and it supports levels from 1 to 9

QAT Memory Requirements
=======================

The user needs to reserve system memory for the QAT memory management to
allocate DMA memory. The size of the reserved system memory depends on the
number of devices used for migration and the number of multifd channels.

Because memory usage depends on QAT configuration, please refer to `QAT Memory
Driver Queries
<https://intel.github.io/quickassist/PG/infrastructure_debugability.html?highlight=memory>`_
for memory usage calculation.

.. list-table:: An example of a PF used for migration
  :header-rows: 1

  * - Number of channels
    - Sender memory usage
    - Receiver memory usage
  * - 2
    - 10M
    - 10M
  * - 4
    - 12M
    - 14M
  * - 8
    - 16M
    - 20M

How To Choose Between QATzip and QPL
====================================
Starting from 4th Gen Intel Xeon Scalable processors, codenamed Sapphire Rapids
processor(``SPR``), multiple built-in accelerators are supported including
``QAT`` and ``IAA``.  The former can accelerate ``QATzip`` and the latter is
used to accelerate ``QPL``.

Here are some suggestions:

1 - If the live migration scenario is limited by network bandwidth and ``QAT``
hardware resources exceed ``IAA``, use the ``QATzip`` method, which can save a
lot of host CPU resources for compression.

2 - If the system cannot support shared virtual memory (SVM) technology, use
the ``QATzip`` method because ``QPL`` performance is not good without SVM
support.

3 - For other scenarios, use the ``QPL`` method first.