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1.0 User-Agent: Mozilla Thunderbird Subject: Re: O_DIRECT vs BLK_FEAT_STABLE_WRITES, was Re: [PATCH] btrfs: never trust the bio from direct IO To: "hch@infradead.org" Cc: Johannes Thumshirn , WenRuo Qu , "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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 qhkIR3qb 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unaccelerated CRC32 is a bad idea. >=20 >> >> I just tried a short hack to always make direct IO to fallback to buffe= red >> IO, the nodatasum performance is the same as the bouncing page solution= , so >> the slow down is not page cache itself but really the checksum. >> >> With CPU features all passed to the VM, the falling-back-to-buffered di= rect >> IO performance is only slightly worse (10~20%) than nodatasum cases. >=20 > I'm a bit lost, what are the exact cases you are comparing here? I'm just checking the impact of btrfs checksum for data writes. I may skipped some points, but overall the idea is: - direct writes, no data csum True zero-copy. - direct writes, no data csum but with bounce pages Affected by memcpy() - direct writes, no data csum but force fallback to buffered Affected by memcpy() - direct writes, data sum Fallback to buffered IO, affected by both memcpy() and csum. So far it looks like the bounce pages solution is wasting a lot of code=20 for almost nothing, it's not any better than falling back. And the checksum part looks can be improved for writes. Since we always fallback to buffered IO for checksums, the content=20 should not change and we can do the submission and checksum calculation=20 in parallel. Already got a prototype, results around 10% improvement inside my VM. Will fix the bugs related to compression and send an RFC for it. Thanks, Qu