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Rapid Containers: Improving zCX Runtime Performance

To improve zCX performance, IBM made multiple enhancements both to the virtualization and networking code that manages the Linux guest

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IBM z/OS® Container Extensions (IBM zCX) makes it possible to integrate Linux® on IBM Z® software applications as Docker Containers within z/OS without requiring a separate Linux server. Application developers can develop and data centers can operate popular open-source packages, Linux applications, IBM software and third-party software together with z/OS applications and data. The ability to colocate the Linux and z/OS parts of a workload improves operational control, leverages z/OS Qualities of Service, and typically improves performance. This is because data can more easily be moved between the zCX address space hosting Linux and the address spaces hosting the z/OS parts.

In order to improve performance of the zCX address space and the Linux applications running inside, IBM has made multiple enhancements both to the virtualization and networking code that manages the Linux guest. The improvements listed in this article address a variety of performance considerations, and should be applied to any z/OS system running zCX. 

High Value zCX Performance Enhancements

Reducing zCX processing contention and switching between GCPs and zIIPs

By reducing the switching between zIIPs and General Purpose processors (GCP), APAR OA58296 provides significant scaling and zIIP eligibility improvements. As zIIP processors always run at full speed even on sub-capacity machines and do not incur software pricing penalties, reducing the time spent on GCPs reduces the total cost of ownership. In general, about 95% of the processing done in a zCX appliance will run on a zIIP processor. OA58296 also reduces contention during virtual CPU and VSAM I/O completion processing. All of these improvements result in much smoother scaling and the ability to reach higher transaction rates. OA58296 is recommended to be applied by all customers.

Allowing Linux to use vector instructions

The IBM Z z13®, z14®, and z15™ processors support Single Instruction Multiple Data (SIMD) instructions, also known as vector instructions. However, it was not until APAR OA59111 that zCX provided vector instruction support to the Linux guest. Vector instructions can dramatically improve the performance of applications that take advantage of them. Applying OA59111 allows Linux applications that utilize these instructions to run in the zCX guest.

Speeding up zCX network communications

The ability to move data in and out of a server is a key performance metric. z/OS Communications Server APARs PH16581 and OA58300 make significant improvements in that area. Their support allows inbound traffic via Workload Queueing (IWQ) when using OSA-Express in QDIO mode. This improves performance and ensures that processing used to manage the traffic will run on zIIP processors to reduce software pricing charges. IWQ is recommended for all zCX users. These APARs also improve the blocking/batching of work elements to improve the processing of zCX traffic.
For more information, see the paper V2R4: Z/OS COMMUNICATIONS SERVER PERFORMANCE SUMMARY REPORT. It provides details on various IBM measurements performed by the Communications Server team with these APARs applied.

Increasing the hardware page frame size to 2G or 1M

By increasing the hardware page frame size that zCX uses to back the Linux guest storage from 4K to either 2G or 1M, APAR OA59573 reduces both zIIP and GCP processing time for the same workload at the same transaction rate. Prior to this support, zCX could only use 4K fixed pages for the Linux guest memory. Using larger page sizes reduces the number of hardware pages that are required, which improves zCX startup and container run-time performance. 2G pages provide the best performance and are recommended for all production environments. However, there are cases where 1M pages are a good choice as well.
As each real storage page requires only one hardware Translation Look-aside Buffer (TLB) entry, larger memory pages can significantly increase the number of TLB hits and thus reduce Dynamic Address Translation (DAT) table lookup processing time that the hardware would otherwise have to do. For example, 4GB of Linux guest storage backed by 4K pages consumes 1,048,576 pages. In contrast, if it was backed by 2G pages, it would only require two pages, and would require significantly less DAT table entries.  

z/OS storage pools

In order to choose a page size and memory capacity plan for the zCX instances, you need to understand how z/OS manages memory and how your choices are related to it. Real memory as of z/OS V2R3 is divided into three different pools:
  1. 2G fixed
  2. 1M and 4K preferred
  3. 1M and 4K non-preferred
The size of the 2G fixed pool is defined at IPL time via the IEASYSxx 2G LFAREA value and cannot be changed after IPL. The remaining memory is divided between the preferred and non-preferred 1M and 4K pools. Within those pools, 1M fixed, 1M pageable and 4K pageable pages can initially be allocated by software. Both 1M pageable and 4K pageable pages can subsequently be fixed. How much is in the non-preferred pool is determined by the IEASYSxx Reconfigurable Storage Units (RSU) value, which can be changed dynamically after IPL. The remaining memory determines the size of the preferred 1M and 4K storage pool.
The IEASYSxx 1M LFAREA value defines the maximum number of 1M fixed pages that can be used across both the 1M and 4K preferred and non-preferred pools. Note that prior to z/OS V2R3, the 1M LFAREA defined a pre-allocated pool like 2G pages. 1M pages are built dynamically and are no longer pre-allocated at IPL. Regardless of zCX planning, changes to the LFAREA values requires an IPL, so consider increasing the 1M value on your next planned IPL. Note that the 1M setting is only a maximum value. Be aware that more 1M fixed usage may appear by address spaces that were previously forced to use 4K fixed pages.
Preferred storage is used to back storage from non-swappable address spaces and for long-term page fixes (as defined by the page fixer). zCX falls into both of those categories. By definition, fixed storage cannot be taken away by either paging or backing it with another available frame. This is because it may be referenced directly by I/O processing that is not coordinated with z/OS Real Storage Manager (RSM). Using non-preferred storage, also known as reconfigurable storage, for such cases would not make sense as that storage must be able to be reconfigured offline. This could be done for various purposes, such as dynamically moving the storage to another LPAR. As memory has become cheaper and LPAR memory sizes have dramatically increased, the need to move memory around on production LPARs is uncommon. Most customers do not need to have much (or any) non-preferred storage defined.

The DISPLAY M=STOR command can be used to display the reconfigurable and non-reconfigurable (preferred storage) values for the system. The F AXR,IAXDMEM command can be used to display the LFAREA defined total sizes, the in-use allocation, and max in-use allocation for 1M fixed and 2G pages (see Figure 1).

Figure 1. z/OS Real Memory Pools 

Choosing a zCX Page Size

Determining the optimal page size for a zCX instance requires planning and consideration. The best performance comes from utilizing a 2G page size, while a 1M size offers good performance. A 4K page size provides the worst performance, especially when running under z/VM.
As the zCX address space is non-swappable and guest storage is fixed for the long term, the real storage used for the guest is not reconfigurable. Thus, using a 1M or 4K page size results in the storage coming from the preferred storage pool. Each zCX instance can be configured with a list of one or more allowable page sizes. Any combination of 2G, 1M and 4K sizes can be used. 
When starting, zCX chooses the page size using a largest size “first-fit” strategy based on the list of chosen sizes for that instance. zCX starts with the largest page size, and then chooses smaller sizes in turn until successful. If none of the page sizes can fit the desired memory size, zCX will terminate with an error message. Choosing multiple page sizes will allow for an instance to start even in the case that the desired page size is unavailable. This can increase availability at the cost of application performance. Always adding 4K to the size list is a good high-availability choice.

zCX appliances provisioned before applying OA59573 will continue to use 4K pages. When starting those appliances, message GLZB026I reminds you to reconfigure them so the ideal size is used. 
How to choose:
  1. Always use 2G for production workloads
  2. Use 1M when the storage needs to be reused by workloads that do not support 2G pages when the zCX appliance is not running
  3. A re-IPL will be required if LFAREA values need to be increased. Until then, 1M or 4K can be used
  4. Consider both in-place and cross-system restarts
  5. Consider adding 4K to the list for highest availability
  6. Always use 1M pages when z/OS is a z/VM guest, as 2G is not supported and 1M provides significant performance improvements
If you’re running zCX on your z/OS systems, you should apply the previously mentioned APARs. In addition, it’s worth spending time reviewing your z/OS and zCX memory allocation settings for improved container performance.
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