Date: Fri, 08 Mar 2013 13:58:38 +0100 From: Andre Oppermann <andre@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: alc@freebsd.org, freebsd-current@freebsd.org, Alan Cox <alc@rice.edu> Subject: Re: Cleanup and untangling of kernel VM initialization Message-ID: <5139E07E.4040704@freebsd.org> In-Reply-To: <20130308091634.GM3794@kib.kiev.ua> References: <510BC24D.2090406@freebsd.org> <510BF6E0.8070007@rice.edu> <5138C877.9060808@freebsd.org> <20130308091634.GM3794@kib.kiev.ua>
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On 08.03.2013 10:16, Konstantin Belousov wrote: > On Thu, Mar 07, 2013 at 06:03:51PM +0100, Andre Oppermann wrote: >> pager_map: is used for pager IO to a storage media (disk). Not >> pageable. Calculation: MAXPHYS * min(max(nbuf/4, 16), 256). > > It is more versatile. The space is used for pbufs, and pbufs currently > also serve for physio, for the clustering, for aio needs. Good to know. Isn't the ceiling of MAXPHYS * 256 a bit tight then? >> memguard_map: is a special debugging submap substituting parts of >> kmem_map. Normally not used. >> >> There is some competition between these maps for physical memory. One >> has to be careful to find a total balance among them wrt. static and >> dynamic physical memory use. > > They mostly compete for KVA, not for the physical memory. Indeed. On 32bit architectures KVA usually is 1GB which is rather limited. >> Within the submaps, especially the kmem_map, we have a number of >> dynamic UMA suballocators where we have to put a ceiling on their >> total memory usage to prevent them to consume all physical *and/or* >> kmem_map virtual memory. This is done with UMA zone limits. > > Note that architectures with the direct maps do not use kmem_map for > the small allocations. The uma_small_alloc() utilizes the direct map > for VA of the new page. kmem_map is needed when allocation is multi-page > sized, to provide the continuous virtual mapping. Can you please explain the direct map some more? I haven't found any good documentation or comments on it. >> It could be that some of the kernel_map submaps are no longer >> necessary and their purpose could simply be emulated by using an >> appropriately limited UMA zone. For example the exec_map is very small >> and only used for the exec arguments. Putting this into pageable >> memory isn't very useful anymore. > > I disagree. Having the strings copied on execve() pageable is good, > the default size of around 260KB max for the strings is quite a > load on the allocator. Oops. You're right. I didn't notice how big ARG_MAX can be. -- Andre
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