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I was wondering if there is going to be large page support for the linux kernel in the near future. I am sure most people know the benefit this has for large databases like Oracle, ultimately larger pages means the TLB slots refer to a much larger area of memory increasing the chance of a TLB hit and avoiding the need to walk the page tables in the MMU. I have spotted a few previous threads where fragmentation and the complexity in making these pages such that they can be evicted were offered as reasons to defer this feature. I think this is a mistake. In Solaris, the first large page support was added to the shared memory segment driver and called Intimate Shared Memory. ISM uses 4MB pages, and is a segment that is locked in memory (non-pagable). Solaris hasn't done anything to address fragmentation as far as I know until recently in Solaris 9 (ISM has been available since 2.6, maybe prior). I have seen many cases where a DBA shuts down an Oracle instance, but cannot restart it because significant memory use has taken place since the instance was shut down (filesystem backed pages read into the page cache, malloc, free, etc..), and in these cases, there was plenty of available memory on the system to handle the request for the ISM segment. In most Oracle installations, the SGA is allocated with an ISM segment that stays resident for entire duration that the Oracle instance is running. This is hardly a case where fragmentation is going to cause an issue since normal 4k or 8k pages are allocated easily in the presence of this segment. To a kernel programmer, the fragmentation and paging issues may seem like a big issue, but any DBA I know would happily make this tradeoff on their production databases for the additional performance that large pages offers. If fragmentation isn't a big deal, and pages can be locked in memory, are large pages still a difficult feature to add to the linux kernel? Lastly, I know I have mentioned Oracle and Solaris a lot. Please don't flame me for this, I think the points I am trying to make are reasonable. Regards, --Buddy - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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