International Journal of Control and Automation
Vol. 29, No. 1,(2016)
Copyright ⓒ 2016 SERSC 447
management, reducing the total write count to NVM should also be taken into
account. Several memory management policies have been proposed for NVM-based
hybrid memory systems.
RaPP [14] uses a modified version of the Multi-Queue [15] algorithm to manage
the pages and try to place performance-critical pages and frequently written pages in
DRAM, which relies on a sophisticated memory controller monitoring access
patterns and ranking pages according to access frequency and write intensity.APG
[16] is a page migration policy aiming at increasing energy efficiency and
minimizing performance degradation. APG maintains access information in page
table entry and moves adaptive number of pages which is decided bythe grouping
technique.
LRU-WPAM[6] uses four monitoring queues (DRAM read queue, DRAM write
queue, NVM read queue and NVM write queue) to monitor the write and read
operations of each pages, so that read-tendency pages are migrated to NVM and
write-tendency pages are migrated to DRAM, but the migrations may promote the
system to evict a page from the buffer and decrease the hit ratio. To this end, APP-
LRU [8] devotes to classifying pages into read-tendency group and write-tendency
group without affecting the hit ratio. When a page fault occurs, APP-LRU decides
whether the missed page is write-tendency or read-tendency based on a history
information table. MHR-LRU [7] records the write operations to DRAM using a
DRAM write list, write heat is judged by the location in the write list. If an empty
DRAM frame is needed, MHR-LRU migrate a write-cold page from DRAM to
NVM. However, both APP-LRU and MHR-LRU migrate pages only when page
faults occur, which cannot migrate pages to the right medium immediately when
access pattern changes.
Another group of memory management policies are designed based on CLOCK
algorithm. Basically, there is no frequently list operation in these algorithms. AIMR
[17] identifies write-intensive pages by virtue of two CLOCK lists, which manages
the pages with “recency” feature and pages with “frequency” feature,
separately.However,since the replacement policy replaces pages which are selected
from one of the CLOCK lists, there is a risk that the replaced pages are warmer than
some pages in the other CLOCK list. Lee et al. proposed CLOCK-DWF [9],
whichimplements an improved CLOCK policy and normal CLOCK policy to
manage DRAM pages and NVM pages, respectively. The improved CLOCK policy
utilizes the frequency and recency of write operations to classify pages. CLOCK-
DWF make sure that all the write operations are performed in DRAM, that means,
the pages should be copied to DRAM before being written. To make room for these
pages, write-cold DRAM pages are migrated to NVM, this action may beperformed
along with an eviction of PCM page. Similarly, D-CLOCK [10] takes the same
mechanism to deal with the write operations. Meanwhile, all the DRAM pages and
NVM pagesare managed by a global CLOCK list, so that all the pages of similar
access pattern have the same chance the be evicted. In addition, in order to limit the
writes to PCM incurred by page faults, D-CLOCK may force to evict a DRAM page
instead of PCM page. These policies put effort into reducing writes to PCM, but
they ignore another important goal of memory management, hit ratio.
3. W-HCLOCK
Higher hit ratio is the most important goal in all of the memory management
policies.Besides, when designingthe memory management policies for NVM-based
hybrid memory system, reducing the total writes to NVM must also be taken into
consideration.