Date: Mon, 1 Mar 2010 21:24:40 -0600 From: Dan Nelson <dnelson@allantgroup.com> To: Shrivatsan <shrivatsan_v@yahoo.com> Cc: freebsd-hackers@freebsd.org, shrivatsan@gmail.com Subject: Re: kernel malloc() and free() Message-ID: <20100302032440.GV70798@dan.emsphone.com> In-Reply-To: <178848.53651.qm@web112006.mail.gq1.yahoo.com> References: <178848.53651.qm@web112006.mail.gq1.yahoo.com>
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In the last episode (Mar 01), Shrivatsan said: > I am looking at the code that allocates and frees kernel memory. I > understand that allocating kernel memory is quite different from the user > level mallocs. In case of user level mallocs, we allocate requested size > + 4 bytes and store the requested size in the additional 4 bytes. Actually FreeBSD's userland malloc hasn't ever had a 4-byte-per-element overhead like this. All BSD mallocs have been block-based, where all objects in a page are the same size. The original bsd malloc had power-of-2 size groupings (blocks holding the same size objects were linked in a list). phkmalloc was imported in 1995 which moves the block metadata into a "page directory", which is separated from the memory returned by malloc(). The current malloc (jemalloc) has a similar setup but scales much better on SMP systems. http://phk.freebsd.dk/pubs/malloc.pdf http://people.freebsd.org/~jasone/jemalloc/bsdcan2006/jemalloc.pdf > However, in the case of kernel, allocating an additional 4 bytes is a > overhead since the request might fall in the next bucket. I looked into > the code and the documentation in the file uma_int.h, but I don't quite > understand the relation between slabs, zones and keg. How do we determine > the size of the memory that we are trying to free from given the virtual > address? I'm not too familiar with the kernel malloc, but it looks like each keg holds objects of the same size (and a keg may hold multiple slabs), so once you get a pointer to the slab header with the vtoslab() macro, slab->us_keg->uk_size points to the size of the allocation. Hm. Even after some reading, I'm not sure how the slabs keep track of which elements are allocated or free. I expected to find a bitmap somewhere that malloc() sets and free() clears, but I don't see it. Maybe some kernel hacker can explain it better :) Regardless, the size of the allocation at this point isn't important, since you know all the items in the page are the same size. -- Dan Nelson dnelson@allantgroup.com
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