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Date: Thu, 05 Jun 2014 08:03:01 -0600 From: Jens Axboe <axboe@...nel.dk> To: Alexander Gordeev <agordeev@...hat.com> CC: linux-kernel@...r.kernel.org, Ming Lei <tom.leiming@...il.com> Subject: Re: blk-mq: bitmap tag: performance degradation? On 2014-06-05 08:01, Alexander Gordeev wrote: > On Wed, Jun 04, 2014 at 08:18:42AM -0600, Jens Axboe wrote: >> A null_blk test is the absolute best case for percpu_ida, since >> there are enough tags and everything is localized. The above test is >> more useful for testing blk-mq than any real world application of >> the tagging. >> >> I've done considerable testing on both 2 and 4 socket (32 and 64 >> CPUs) and bitmap tagging is better in a much wider range of >> applications. This includes even high tag depth devices like nvme, >> and more normal ranges like mtip32xx and scsi-mq setups. > > Just for the record: bitmap tags on a 48 CPU box with NVMe device > indeed shows almost the same performance/cache rate as the stock > kernel. Thanks for confirming. It's one of the dangers of null_blk, it's not always a very accurate simulation of what a real device will do. I think it's mostly a completion side thing, would be great with a small device that supported msi-x and could be used as an irq trigger :-) -- Jens Axboe -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.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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