Messages in this thread | | | Subject | Re: [RFC][PATCH] ipc: Remove IPCMNI | From | Manfred Spraul <> | Date | Thu, 29 Mar 2018 20:07:44 +0200 |
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Hello Mathew,
On 03/29/2018 12:56 PM, Matthew Wilcox wrote: > On Thu, Mar 29, 2018 at 10:47:45AM +0200, Manfred Spraul wrote: >>>>>>> This can be implemented trivially with the current code >>>>>>> using idr_alloc_cyclic. >> Is there a performance impact? >> Right now, the idr tree is only large if there are lots of objects. >> What happens if we have only 1 object, with id=INT_MAX-1? > The radix tree uses a branching factor of 64 entries (6 bits) per level. > The maximum ID is 31 bits (positive signed 32-bit integer). So the > worst case for a single object is 6 pointer dereferences to find the > object anywhere in the range (INT_MAX/2 - INT_MAX]. That will read 12 > cachelines. If we were to constrain ourselves to a maximum of INT_MAX/2 > (30 bits), we'd reduce that to 5 pointer dereferences and 10 cachelines. I'm concerned about the up to 6 branches. But this is just guessing, we need a test with a realistic workload.
-- Manfred
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