Old Linux patch slows down AMD Ryzen and Epyc

Old Linux patch slows down AMD Ryzen and Epyc

An AMD developer has provided a Linux kernel patch that eliminates a legacy issue: queues when switching to ACPI C2 and C3 power saving modes. These “phantom processes” slow down existing AMD processors in Linux during certain data transfers.

“Mock OP” queues were created in the ACPI Power Management kernel drivers in March 2002 in order to make some Athlon computers more economical. Because at that time, AMD Athlon had an error regarding some chipset (VIA): The STPCLK# signal, which controls turning off the clock signal, did not work as expected. Thus, additional “Dummy OP” accesses waste some time before the system can safely switch to deeper sleep. In order to save electricity and reduce wasted heat despite Athlon’s sleep disturbance, additional software tools were also used under Windows and Linux in 2001.

Apparently, however, the Linux patch has fallen into oblivion. As early as 2006, someone complained about the lack of documentation. Programmer K Prateek Nayak has now introduced a new patch that disables the old patch for new AMD processors. Using the “tbench” standard as an example, it shows that phantom operations can reduce data transfer rate drastically, but fortunately only in special cases: that is, when the processor has to wake up from sleep to complete a data transfer task.

According to the site Voronex Should the new drivers/acpi/processor_idle.c patch come with kernel 6.0.

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