Os Memory Pagemap
DodaTech
1 min read
In this tutorial, you'll learn about How to Fix Pagemap Errors. We cover key concepts, practical examples, and best practices.
Fix pagemap errors when page table entries not checked causing high memory usage misunderstanding.
Quick Fix
Wrong
import os
# RSS vs VMEM confusion:
data=[0]*10**8
print(f'Virtual: {len(data)*8//1024//1024} MB') # 800MB virtual
# RSS may be much less (pages not touched yet)
Process shows 800MB virtual in ps but only 10MB RSS. Actual RAM usage unclear.
Right
import os
# Read /proc/PID/pagemap to check which pages are in RAM:
def is_page_present(pid,vaddr):
with open(f'/proc/{pid}/pagemap','rb') as f:
page_size=4096
entry_size=8
offset=(vaddr//page_size)*entry_size
f.seek(offset)
entry=int.from_bytes(f.read(entry_size),'little')
present=entry & (1<<63)
swapped=entry & (1<<62)
pfn=entry & ((1<<55)-1)
return present>0, swapped>0, pfn
# Check process RSS:
with open(f'/proc/{os.getpid()}/status') as f:
for line in f:
if 'VmRSS' in line: print(line.strip())
if 'VmSize' in line: print(line.strip())
VmRSS: 5MB, VmSize: 800MB. Only touched pages use RAM.
Prevention
Use /proc/PID/pagemap for page-level presence info. RSS != virtual memory.
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