Date: Sun, 2 Oct 2011 17:45:45 +0400 From: Lev Serebryakov <lev@FreeBSD.org> To: Davide Italiano <davide.italiano@gmail.com> Cc: freebsd-hackers@freebsd.org, lev@freebsd.org Subject: Re: Memory allocation in kernel -- what to use in which situation? What is the best for page-sized allocations? Message-ID: <1393358703.20111002174545@serebryakov.spb.ru> In-Reply-To: <CACYV=-FNM-3fcYzFGc9eFajdoBmG1E-rWo6tq-OwBefGPADywA@mail.gmail.com> References: <358651269.20111002162109@serebryakov.spb.ru> <CACYV=-FNM-3fcYzFGc9eFajdoBmG1E-rWo6tq-OwBefGPADywA@mail.gmail.com>
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Hello, Davide. You wrote 2 =EE=EA=F2=FF=E1=F0=FF 2011 =E3., 16:57:48: >> =A0 But what if I need to allocate a lot (say, 16K-32K) of page-sized >> blocks? Not in one chunk, for sure, but in lifetime of my kernel >> module. Which allocator should I use? It seems, the best one will be >> very low-level only-page-sized allocator. Is here any in kernel? > My 2cents: > Everytime you request a certain amount of memory bigger than 4KB using > kernel malloc(), it results in a direct call to uma_large_malloc(). > Right now, uma_large_malloc() calls kmem_malloc() (i.e. the memory is > requested to the VM directly). > This kind of approach has two main drawbacks: > 1) it heavily fragments the kernel heap > 2) when free() is called on these multipage chunks, it in turn calls > uma_large_free(), which immediately calls the VM system to unmap and > free the chunk of memory. The unmapping requires a system-wide TLB > shootdown, i.e. a global action by every processor in the system. > I'm currently working supervised by alc@ to an intermediate layer that > sits between UMA and the VM, which goal is satisfyinh efficiently requests >> 4KB (so, the one you want considering you're asking for > 16KB-32KB), but the work is in an early stage. I was not very clear here. I'm saying about page-sized blocks, but many of them. 16K-32K is not a size in bytes, but count of page-sized blocks my code needs :) BTW, I/O is often require big buffers, up to MAXPHYS (128KiB for now), do you mean, that any allocation of such memory has considerable performance penalties, especially on multi-core and multi-CPU systems? --=20 // Black Lion AKA Lev Serebryakov <lev@FreeBSD.org>
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