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Os Syscall Vdso

DodaTech 1 min read

In this tutorial, you'll learn about How to Fix vDSO Errors. We cover key concepts, practical examples, and best practices.

Fix vdso errors when syscall to gettimeofday uses kernel entry instead of vDSO (10x slower).

Quick Fix

Wrong

import os, time, ctypes
# Slow way: syscall each time
def slow_time():
    return os.times()  # syscall!
s=time.perf_counter()
for _ in range(10000): slow_time()
print(f'Syscall: {time.perf_counter()-s:.3f}s')
def fast_time():
    return time.time_ns()  # uses vDSO (no syscall)

gettimeofday via syscall: ~100ns per call. Same via vDSO: ~10ns. 10x difference.

import os, time
# Use time functions that go through vDSO:
# time.time() - uses vDSO gettimeofday (no syscall)
# time.time_ns() - uses vDSO clock_gettime
# time.clock_gettime() - uses vDSO
s=time.perf_counter()
for _ in range(10000):
    t=time.time_ns()  # vDSO, no syscall
print(f'vDSO: {time.perf_counter()-s:.3f}s')
# Check vDSO mappings:
with open('/proc/self/maps') as f:
    for line in f:
        if '[vdso]' in line: print(f'vDSO mapped: {line.strip()}')

time.time_ns() uses vDSO. 10x faster than syscall for time-related operations.

Prevention

time.time() and time.time_ns() use vDSO. Avoid os.times() for frequent time queries.

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FAQ

What is vDSO?

Virtual dynamic shared object. Kernel maps pages with syscall implementations in userspace.

Which calls?

gettimeofday, clock_gettime (monotonic/realtime), getcpu. No kernel transition needed.

Perf advantage?

Syscall: ~100ns. vDSO: ~10ns. Significant for high-frequency timestamps.

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