stefanB stefanB - 1 month ago 23
C Question

mmap for writing sequential log file for speed?

I want to write log file, unstructured format (one line at a time), using

(for speed). What is the best procedure? Do I open empty file,
to 1 page size (write empty string to resize file?), then
- and repeat when mmaped area full?

I usually use
for writing fixed size structures, usually just one page at a time, however this is for writing log files (anywhere from 0.5 - 10 Gb) using mmap but not sure what's the best practice once the first mmaped area is filled -
, resize file
next page ?

While writing logs to memory area, I would tracking size, and
, what is the proper handling once I get to the end of the mapped memory area?

Let's say I never need to go back or overwrite existing data, so I only write new data to file.

Q1: When I get to the end of mapped area do I
file to resize by another page size and
the next page ?

Q2: Is there a standard way to pre-empt and have the next page ready in memory for next write? Do this on another thread when we get close to the end of mapped area ?

Q3: Do I
for sequential access?

This is for real time data processing with requirement to keep log file - currently I just write to file. Log file is unstructured, text format, line based.

This is for linux/c++/c optionally testing on Mac (so no remap [?]).

Any links/pointers to best practices appreciated.


I wrote my bachelor thesis about the comparism of fwrite VS mmap ("An Experiment to Measure the Performance Trade-off between Traditional I/O and Memory-mapped Files"). First of all, for writing, you don't have to go for memory-mapped files, espacially for large files. fwrite is totally fine and will nearly always outperform approaches using mmap. mmap will give you the most performance boosts for parallel data reading; for sequential data writing your real limitation with fwrite is your hardware.

In my examples remapSize is the initial size of the file and the size by which the file gets increased on each remapping. fileSize keeps track of the size of the file, mappedSpace represents the size of the current mmap (it's length), alreadyWrittenBytes are the bytes that have already been written to the file.

Here is the example initalization:

void init() {
  fileDescriptor = open(outputPath, O_RDWR | O_CREAT | O_TRUNC, (mode_t) 0600); // Open file
  result = ftruncate(fileDescriptor, remapSize); // Init size
  fsync(fileDescriptor); // Flush
  memoryMappedFile = (char*) mmap64(0, remapSize, PROT_WRITE, MAP_SHARED, fileDescriptor, 0); // Create mmap
  fileSize = remapSize; // Store mapped size
  mappedSpace = remapSize; // Store mapped size

Ad Q1:

I used an "Unmap-Remap"-mechanism.


  • first flushes (msync)
  • and then unmaps the memory-mapped file.

This could look the following:

void unmap() {
  msync(memoryMappedFile, mappedSpace, MS_SYNC); // Flush
  munmap(memoryMappedFile, mappedSpace)

For Remap, you have the choice to remap the whole file or only the newly appended part.

Remap basically

  • increases the file size
  • creates the new memory map

Example implementation for a full remap:

void fullRemap() {
  ftruncate(fileDescriptor, mappedSpace + remapSize); // Make file bigger
  fsync(fileDescriptor); // Flush file
  memoryMappedFile = (char*) mmap64(0, mappedSpace + remapSize, PROT_WRITE, MAP_SHARED, fileDescriptor, 0); // Create new mapping on the bigger file
  fileSize += reampSize;
  mappedSpace += remapSize; // Set mappedSpace to new size

Example implementation for the small remap:

void smallRemap() {
  ftruncate(fileDescriptor, fileSize + remapSize); // Make file bigger
  fsync(fileDescriptor); // Flush file
  remapAt = alreadyWrittenBytes % pageSize == 0 
            ? alreadyWrittenBytes 
            : alreadyWrittenBytes - (alreadyWrittenBytes % pageSize); // Adjust remap location to pagesize
  memoryMappedFile = (char*) mmap64(0, fileSize + remapSize - remapAt, PROT_WRITE, MAP_SHARED, fileDescriptor, remapAt); // Create memory-map
  fileSize += remapSize;
  mappedSpace = fileSize - remapAt;

There is a mremap function out there, yet it states

This call is Linux-specific, and should not be used in programs intended to be portable.

Ad Q2:

I'm not sure if I understood that point right. If you want to tell the kernel "and now load the next page", then no, this is not possible (at least to my knowledge). But see Ad Q3 on how to advise the kernel.

Ad Q3:

You can use madvise with the flag MADV_SEQUENTIAL, yet keep in mind that this does not enforce the kernel to read ahead, but only advices it.

Excerp form the man:

This may cause the kernel to aggressively read-ahead

Personal conclusion:

Do not use mmap for sequential data writing. It will just cause much more overhead and will lead to much more "unnatural" code than a simple writing alogrithm using fwrite.

Use mmap for random access reads to large files.

This are also the results that were obtained during my thesis. I was not able to achieve any speedup by using mmap for sequential writing, in fact, it was always slower for this purpose.