XCOM receive use of zEDC decompression is MAXPACK dependent

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Article ID: 237346

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Products

XCOM Data Transport - z/OS XCOM Data Transport XCOM Data Transport - Windows XCOM Data Transport - Linux PC

Issue/Introduction

Testing use of zEDC it has been observed that when a receiving server is configured MAXPACK=1023000, zEDC is not used for decompression unless the sending side also uses MAXPACK=1023000.
In the uploaded traces file:
 - the first ZLIB transfer was sent MAXPACK=31744, zEDC was used for compression on the sending side while software decompression was used on the receiving side.
 - the second ZLIB transfer was sent with MAXPACK=1023000, and zEDC compression/decompression was used on both sides.

Environment

Release : 12.0

Component : XCOM Data Transport for z/OS

Resolution

In the traces sent, the compressed data buffers vary between about 2500 - 2800 bytes each.
The XCOM code currently enforces a 4KB minimum to use zEDC, which is why the decompression process consistently is falling back to software instead of zEDC.
NOTE: It is not possible to influence the size of the compressed buffer very much. The size of the data before compression as well as the content of the data itself are the main factors that govern to what extent data gets compressed, and its resultant size.

XCOM Engineering has made a code change to reduce the 4KB limit to the hardware-enabled minimum of 1KB which will be available in LU05222 ("REDUCE MINIMUM BUFFER REQUIREMENTS FOR ZEDC").

Additional Information

For many of Engineering's tests in-house on R&D systems, the compressed output is less than 1KB . In those cases, the decompression ALWAYS falls back to software, rather than zEDC.  There is nothing that can be done about that unless IBM changes their interface. So some data may potentially be out-of-band for zEDC processing.  That is strictly dependent on the actual data being compressed and how well it lends itself to compression.
There is nothing XCOM can do about that limitation and zEDC will NEVER be used for buffers that compress to < 1KB .