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Re: O_DIRECT vs BLK_FEAT_STABLE_WRITES, was Re: [PATCH] btrfs: never trust the bio from direct IO To: Johannes Thumshirn , WenRuo Qu , "hch@infradead.org" Cc: "linux-btrfs@vger.kernel.org" , "djwong@kernel.org" , "linux-xfs@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-mm@kvack.org" , "martin.petersen@oracle.com" , "jack@suse.com" References: <4f4c468a-ac87-4f54-bc5a-d35058e42dd2@suse.com> <25742d91-f82e-482e-8978-6ab2288569da@wdc.com> Content-Language: en-US From: Qu Wenruo Autocrypt: addr=quwenruo.btrfs@gmx.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB 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 STs48RJW 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d on >>>> XFS? >>> If the bandwith of the SSDs get close or exceeds the DRAM bandwith >>> buffered I/O can be 50% or less of the direct I/O performance. >> In my case, the DRAM is way faster than the SSD (tens of GiB/s vs less >> than 5GiB/s). >> >>>> With this patch I'm able to enable direct IO for inodes with checksum= s. >>>> I thought it would easily improve the performance, but the truth is, = it's >>>> not that different from buffered IO fall back. >>> That's because you still copy data. >> Enabling the extra copy for direct IO only drops around 15~20% >> performance, but that's on no csum case. >> >> So far the calculation matches your estimation, but... >> >>>> So I start wondering if it's the checksum itself causing the miserabl= e >>>> performance numbers. >>> Only indirectly by touching all the cachelines. But once you copy you >>> touch them again. Especially if not done in small chunks. >> As long as I enable checksum verification, even with the bouncing page >> direct IO, the result is not any better than buffered IO fallback, all >> around 10% (not by 10%, at 10%) of the direct IO speed (no matter >> bouncing or not). >> >> Maybe I need to check if the proper hardware accelerated CRC32 is >> utilized... >=20 >=20 > You could also hack in a NULL-csum for testing. Something that writes a > fixed value every time. This would then rule out all the cost of the > csum generation and only test the affected IO paths. >=20 It turns out to be checksum, and particularly my VM setup. My VM is using kvm64 CPU type, which blocks quite a lot of CPU features,= =20 thus the CRC32 performance is pretty poor. I just tried a short hack to always make direct IO to fallback to=20 buffered IO, the nodatasum performance is the same as the bouncing page=20 solution, so the slow down is not page cache itself but really the checksu= m. With CPU features all passed to the VM, the falling-back-to-buffered=20 direct IO performance is only slightly worse (10~20%) than nodatasum cases= . Really sorry for the noise. Thanks, Qu