Memory allocation performance

Alexander Motin mav at FreeBSD.org
Thu Jan 31 15:04:59 PST 2008


Hi.

While profiling netgraph operation on UP HEAD router I have found that 
huge amount of time it spent on memory allocation/deallocation:

         0.14  0.05  132119/545292      ip_forward <cycle 1> [12]
         0.14  0.05  133127/545292      fxp_add_rfabuf [18]
         0.27  0.10  266236/545292      ng_package_data [17]
[9]14.1 0.56  0.21  545292         uma_zalloc_arg [9]
         0.17  0.00  545292/1733401     critical_exit <cycle 2> [98]
         0.01  0.00  275941/679675      generic_bzero [68]
         0.01  0.00  133127/133127      mb_ctor_pack [103]

         0.15  0.06  133100/545266      mb_free_ext [22]
         0.15  0.06  133121/545266      m_freem [15]
         0.29  0.11  266236/545266      ng_free_item [16]
[8]15.2 0.60  0.23  545266         uma_zfree_arg [8]
         0.17  0.00  545266/1733401     critical_exit <cycle 2> [98]
         0.00  0.04  133100/133100      mb_dtor_pack [57]
         0.00  0.00  134121/134121      mb_dtor_mbuf [111]

I have already optimized all possible allocation calls and those that 
left are practically unavoidable. But even after this kgmon tells that 
30% of CPU time consumed by memory management.

So I have some questions:
1) Is it real situation or just profiler mistake?
2) If it is real then why UMA is so slow? I have tried to replace it in 
some places with preallocated TAILQ of required memory blocks protected 
by mutex and according to profiler I have got _much_ better results. 
Will it be a good practice to replace relatively small UMA zones with 
preallocated queue to avoid part of UMA calls?
3) I have seen that UMA does some kind of CPU cache affinity, but does 
it cost so much that it costs 30% CPU time on UP router?

Thanks!

-- 
Alexander Motin


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