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Date: Fri, 01 Feb 2008 00:30:09 +0100
From: Kris Kennaway <kris@FreeBSD.org>
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To: Alexander Motin <mav@FreeBSD.org>
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Cc: freebsd-hackers@freebsd.org, freebsd-performance@freebsd.org
Subject: Re: Memory allocation performance
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Alexander Motin wrote:
> 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?

Make sure you have INVARIANTS disabled, it has a high performance cost 
in UMA.

Kris