Summary

When Intel® launched their Optane™ persistent memory (PMEM) modules in 2019, we were eager to understand how they would perform when mixed with traditional DRAM DIMMs. Now that sufficient testing and analysis has been completed, we know that both memory types, PMEM and DRAM, have similar performance readings. This means that PowerEdge customers can increase total capacity or reduce TCO without impacting the total system performance. This DfD will discuss our test study conducted for PMEM and DRAM performance readings, and explain what opportunities this creates for PowerEdge customers.

Introduction

Dell Technologies offers support for Intel® Optane™ PMEM (Persistent Memory) for previous-generation and current-generation PowerEdge servers. This support for mixing PMEM with DRAM allows customers to increase their total memory capacity or lower TCO, while having memory persistence in application direct mode.

However, customers were concerned that mixing DRAM and PMEM would cause performance degradation. Dell Technologies decided to conduct internal testing to better understand if there is a negative performance impact when these two memory types are mixed.

Test Setup

A 4-socket PowerEdge R940 was configured with the following:

• 4 Intel Xeon 8280L CPUs
• PERC H740P adapter with 8GB of cache
• OS on 5 1.8TB HDDs (RAID 5)
• HANA on 12x800GB SAS SSD (RAID 5)
• SLES 15 OS
• HANA 2.0 Revision 41

As for the memory populated, the PowerEdge R940 supports up to 48 memory slots; 24 of which support PMEM drives. Therefore, the first 24 memory slots consist of 128GB DRAM, and the remaining 24 memory slots were divided into 3 test cases:

• Test Case 1: 128 GB DRAM
• Test Case 2: 128 GB PMEM
• Test Case 3: 512 GB PMEM

Results were achieved with SAP BW Edition for performance test based on the scenario defined in the SAP BW edition for SAP HANA Standard Application Benchmark [sap.com]. Measurements from 5+ test runs were normalized to avoid fluctuation.

Test Results – Relative Performance

Two test phases were performed. The first test phase measured the completion times required to load test data from the drives to the database. The times required to load over 2.5 billion records into an empty database were nearly identical, with PMEM taking only 1.8% longer to load than DRAM. This establishes that the load speeds of both memory types are nearly identical.

The second test phase measured the number of queries per hour performed in SAP HANA. As seen in Figure 1, the configuration loaded with 100% 128GB DRAM performed the best and is our baseline for PMEM comparison. The next

two configurations with 50% 128GB PMEM and 50% 512GB PMEM trailed closely behind, with only a -8.89% and -7.56% performance hit for each PMEM drive config, respectively. These readings establish that DRAM and PMEM have similar performance, so customers investigating PMEM as an alternative to DRAM should only see marginal performance degradation.

Figure 1 – Bar graph illustrating queries per hour in relative percentages for 128GB DRAM, 128GB PMEM and 512GB PMEM

PMEM Advantages

 Reduce TCO

Customers can explore mixing DRAM and PMEM quantities to optimize the $/GB for their server needs. DRAM typically ranges from $41/GB - $64/GB, while PMEM ranges from $12/GB - $20GB, based on capacity size and current market pricing. This means that customers can reduce costs by up to 71% when populating the open 24 memory slots with supported PMEM modules! Figure 2 below illustrates the $/GB of various mixed DRAM and PMEM configurations in the PE R940:

Figure 2 – Bar graph illustrating the $/GB and total capacities of various mixed DRAM and PMEM configurations on the PowerEdge R940

Increase Memory Capacity

Because PMEM has higher capacities than DRAM, users can boost their total memory capacity by populating their memory slots with 256GB or 512GB PMEM. In the case of our PE R940 test study, 24 memory slots are already occupied by 128GB DRAM. So, if the remaining 24 memory slots are populated with 256GB or 512GB PMEM, customers would see capacity gains of up to 50% and 150%, respectively.

Increase Memory Capacity AND Reduce TCO

Even more enticing is the fact that customers can increase their memory capacity, while also lowering their TCO. There are many mixed memory configurations that will give this “best of both worlds” return, but Figure 3 below shows one example on the PE R940. By replacing 24 of the 128GB DRAM with 512GB PMEM, users can increase total memory capacity by up to 150% while reducing memory TCO by up to 41.6%. Again, please keep in mind that TCO percentages are subject to change depending on capacity size and the current market pricing. 

Figure 3 – Bar graph illustrating the $/GB and total capacities of various mixed DRAM and PMEM configurations on the PowerEdge R940

Conclusion

By conducting studies that highlight the performance impact of mixing DRAM and PMEM, Dell Technologies was able to conclude that the performance of each memory type was very similar, with nearly identical load times and only a slight performance hit for the number of queries run in SAP HANA. These findings provide customers adequate reassurance to invest in PMEM modules that can increase memory capacity, reduce TCO, or even both!