[Bugme-new] [Bug 17531] New: (arch_)get_unmapped_area can be terribly slow due to unnecessary linear search and find_vma

bugzilla-daemon at bugzilla.kernel.org bugzilla-daemon at bugzilla.kernel.org
Mon Aug 30 13:50:13 PDT 2010


           Summary: (arch_)get_unmapped_area can be terribly slow due to
                    unnecessary linear search and find_vma
           Product: Memory Management
           Version: 2.5
    Kernel Version: 2.6.35
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: Other
        AssignedTo: akpm at linux-foundation.org
        ReportedBy: luca.barbieri at gmail.com
        Regression: No

Currently most/all versions of get_unmapped_area perform a linear search in the
process virtual address space to find free space.

Some, like arch_get_unmapped_area_topdown for x86-64 even call find_vma for
each step, which does a full walk on the rb-tree of vmas.

Instead, they should use, from slower to faster:
- O(n) but faster: a linked list of virtual address space holes
- O(log(n)): an rb-tree of virtual address space holes indexed by size
- O(1): a buddy allocator of virtual address space holes, or another scheme
with buckets

Is there any reason this issue hasn't been fixed yet? (i.e. any reason none of
the proposed schemes are feasible?)

Workloads doing a lot of mmaps tend to suffer greatly, especially on the
versions that do a find_vma for each step of the scan.

An example are OpenGL drivers using DRM/GEM/TTM who don't employ userspace
caching and suballocation of TTM allocated buffers.

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