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Mon, 24 Aug 2020 06:32:55 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id B191FB1F0; Mon, 24 Aug 2020 10:33:23 +0000 (UTC) Subject: Re: [PATCH v2 1/3] btrfs: change how we calculate the nrptrs for btrfs_buffered_write() To: Qu Wenruo , linux-btrfs@vger.kernel.org References: <20200824075959.85212-1-wqu@suse.com> <20200824075959.85212-2-wqu@suse.com> From: Nikolay Borisov Autocrypt: addr=nborisov@suse.com; prefer-encrypt=mutual; keydata= xsFNBFiKBz4BEADNHZmqwhuN6EAzXj9SpPpH/nSSP8YgfwoOqwrP+JR4pIqRK0AWWeWCSwmZ T7g+RbfPFlmQp+EwFWOtABXlKC54zgSf+uulGwx5JAUFVUIRBmnHOYi/lUiE0yhpnb1KCA7f u/W+DkwGerXqhhe9TvQoGwgCKNfzFPZoM+gZrm+kWv03QLUCr210n4cwaCPJ0Nr9Z3c582xc bCUVbsjt7BN0CFa2BByulrx5xD9sDAYIqfLCcZetAqsTRGxM7LD0kh5WlKzOeAXj5r8DOrU2 GdZS33uKZI/kZJZVytSmZpswDsKhnGzRN1BANGP8sC+WD4eRXajOmNh2HL4P+meO1TlM3GLl EQd2shHFY0qjEo7wxKZI1RyZZ5AgJnSmehrPCyuIyVY210CbMaIKHUIsTqRgY5GaNME24w7h TyyVCy2qAM8fLJ4Vw5bycM/u5xfWm7gyTb9V1TkZ3o1MTrEsrcqFiRrBY94Rs0oQkZvunqia c+NprYSaOG1Cta14o94eMH271Kka/reEwSZkC7T+o9hZ4zi2CcLcY0DXj0qdId7vUKSJjEep c++s8ncFekh1MPhkOgNj8pk17OAESanmDwksmzh1j12lgA5lTFPrJeRNu6/isC2zyZhTwMWs k3LkcTa8ZXxh0RfWAqgx/ogKPk4ZxOXQEZetkEyTFghbRH2BIwARAQABzSJOaWtvbGF5IEJv cmlzb3YgPG5ib3Jpc292QHN1c2UuZGU+wsF4BBMBAgAiBQJYijkSAhsDBgsJCAcDAgYVCAIJ CgsEFgIDAQIeAQIXgAAKCRBxvoJG5T8oV/B6D/9a8EcRPdHg8uLEPywuJR8URwXzkofT5bZE IfGF0Z+Lt2ADe+nLOXrwKsamhweUFAvwEUxxnndovRLPOpWerTOAl47lxad08080jXnGfYFS Dc+ew7C3SFI4tFFHln8Y22Q9075saZ2yQS1ywJy+TFPADIprAZXnPbbbNbGtJLoq0LTiESnD w/SUC6sfikYwGRS94Dc9qO4nWyEvBK3Ql8NkoY0Sjky3B0vL572Gq0ytILDDGYuZVo4alUs8 LeXS5ukoZIw1QYXVstDJQnYjFxYgoQ5uGVi4t7FsFM/6ykYDzbIPNOx49Rbh9W4uKsLVhTzG BDTzdvX4ARl9La2kCQIjjWRg+XGuBM5rxT/NaTS78PXjhqWNYlGc5OhO0l8e5DIS2tXwYMDY LuHYNkkpMFksBslldvNttSNei7xr5VwjVqW4vASk2Aak5AleXZS+xIq2FADPS/XSgIaepyTV tkfnyreep1pk09cjfXY4A7qpEFwazCRZg9LLvYVc2M2eFQHDMtXsH59nOMstXx2OtNMcx5p8 0a5FHXE/HoXz3p9bD0uIUq6p04VYOHsMasHqHPbsMAq9V2OCytJQPWwe46bBjYZCOwG0+x58 fBFreP/NiJNeTQPOa6FoxLOLXMuVtpbcXIqKQDoEte9aMpoj9L24f60G4q+pL/54ql2VRscK d87BTQRYigc+ARAAyJSq9EFk28++SLfg791xOh28tLI6Yr8wwEOvM3wKeTfTZd+caVb9gBBy wxYhIopKlK1zq2YP7ZjTP1aPJGoWvcQZ8fVFdK/1nW+Z8/NTjaOx1mfrrtTGtFxVBdSCgqBB jHTnlDYV1R5plJqK+ggEP1a0mr/rpQ9dFGvgf/5jkVpRnH6BY0aYFPprRL8ZCcdv2DeeicOO YMobD5g7g/poQzHLLeT0+y1qiLIFefNABLN06Lf0GBZC5l8hCM3Rpb4ObyQ4B9PmL/KTn2FV Xq/c0scGMdXD2QeWLePC+yLMhf1fZby1vVJ59pXGq+o7XXfYA7xX0JsTUNxVPx/MgK8aLjYW hX+TRA4bCr4uYt/S3ThDRywSX6Hr1lyp4FJBwgyb8iv42it8KvoeOsHqVbuCIGRCXqGGiaeX Wa0M/oxN1vJjMSIEVzBAPi16tztL/wQtFHJtZAdCnuzFAz8ue6GzvsyBj97pzkBVacwp3/Mw qbiu7sDz7yB0d7J2tFBJYNpVt/Lce6nQhrvon0VqiWeMHxgtQ4k92Eja9u80JDaKnHDdjdwq FUikZirB28UiLPQV6PvCckgIiukmz/5ctAfKpyYRGfez+JbAGl6iCvHYt/wAZ7Oqe/3Cirs5 KhaXBcMmJR1qo8QH8eYZ+qhFE3bSPH446+5oEw8A9v5oonKV7zMAEQEAAcLBXwQYAQIACQUC WIoHPgIbDAAKCRBxvoJG5T8oV1pyD/4zdXdOL0lhkSIjJWGqz7Idvo0wjVHSSQCbOwZDWNTN JBTP0BUxHpPu/Z8gRNNP9/k6i63T4eL1xjy4umTwJaej1X15H8Hsh+zakADyWHadbjcUXCkg OJK4NsfqhMuaIYIHbToi9K5pAKnV953xTrK6oYVyd/Rmkmb+wgsbYQJ0Ur1Ficwhp6qU1CaJ mJwFjaWaVgUERoxcejL4ruds66LM9Z1Qqgoer62ZneID6ovmzpCWbi2sfbz98+kW46aA/w8r 7sulgs1KXWhBSv5aWqKU8C4twKjlV2XsztUUsyrjHFj91j31pnHRklBgXHTD/pSRsN0UvM26 lPs0g3ryVlG5wiZ9+JbI3sKMfbdfdOeLxtL25ujs443rw1s/PVghphoeadVAKMPINeRCgoJH zZV/2Z/myWPRWWl/79amy/9MfxffZqO9rfugRBORY0ywPHLDdo9Kmzoxoxp9w3uTrTLZaT9M KIuxEcV8wcVjr+Wr9zRl06waOCkgrQbTPp631hToxo+4rA1jiQF2M80HAet65ytBVR2pFGZF zGYYLqiG+mpUZ+FPjxk9kpkRYz61mTLSY7tuFljExfJWMGfgSg1OxfLV631jV1TcdUnx+h3l Sqs2vMhAVt14zT8mpIuu2VNxcontxgVr1kzYA/tQg32fVRbGr449j1gw57BV9i0vww== Message-ID: Date: Mon, 24 Aug 2020 13:32:53 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200824075959.85212-2-wqu@suse.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org On 24.08.20 г. 10:59 ч., Qu Wenruo wrote: > @nrptrs of btrfs_bufferd_write() determines the up limit of pages we can > process in one batch. > > Normally we want it to be as large as possible as btrfs metadata/data > reserve and release can cost quite a lot of time. > > Commit 142349f541d0 ("btrfs: lower the dirty balance poll interval") > introduced two extra limitations which are suspicious now: > - limit the page number to nr_dirtied_pause - nr_dirtied > However I can't find any mainline fs nor iomap utilize these two > members. > Although page write back still uses those two members, as no other fs > utilizeing them at all, I doubt about the usefulness. > > - ensure we always have 8 pages allocates > The 8 lower limit looks pretty strange, this means even we're just > writing 4K, we will allocate page* for 8 pages no matter what. > To me, this 8 pages look more like a upper limit. > > This patch will change it by: > - Extract the calculation into another function > This allows us to add more comment explaining every calculation. > > - Do proper page alignment calculation > The old calculation, DIV_ROUND_UP(iov_iter_count(i), PAGE_SIZE) > doesn't take @pos into consideration. > In fact we can easily have iov_iter_count(i) == 2, but still cross two > pages. (pos == page_offset() + PAGE_SIZE - 1). > > - Remove the useless max(8) > > - Use PAGE_SIZE independent up limit > Now we use 64K as nr_pages limit, so we should get similar performance > between different arches. > > Signed-off-by: Qu Wenruo > --- > fs/btrfs/file.c | 28 ++++++++++++++++++++++++---- > 1 file changed, 24 insertions(+), 4 deletions(-) > > diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c > index 5a818ebcb01f..c592350a5a82 100644 > --- a/fs/btrfs/file.c > +++ b/fs/btrfs/file.c > @@ -1620,6 +1620,29 @@ void btrfs_check_nocow_unlock(struct btrfs_inode *inode) > btrfs_drew_write_unlock(&inode->root->snapshot_lock); > } > > +/* Helper to get how many pages we should alloc for the batch */ > +static int get_nr_pages(struct btrfs_fs_info *fs_info, loff_t pos, > + struct iov_iter *iov) Drop the fs_info parameter as it's unsued. Also rename the function to calc_nr_pages since it's closer to what the function is actually doing. > +{ > + int nr_pages; > + > + /* > + * Try to cover the full iov range, as btrfs metadata/data reserve > + * and release can be pretty slow, thus the more pages we process in > + * one batch the better. > + */ > + nr_pages = (round_up(pos + iov_iter_count(iov), PAGE_SIZE) - > + round_down(pos, PAGE_SIZE)) / PAGE_SIZE; > + > + /* > + * But still limit it to 64KiB, so we can still get a similar > + * buffered write performance between different page sizes > + */ > + nr_pages = min_t(int, nr_pages, SZ_64K / PAGE_SIZE); > + > + return nr_pages; > +} > + > static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb, > struct iov_iter *i) > { > @@ -1638,10 +1661,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb, > bool only_release_metadata = false; > bool force_page_uptodate = false; > > - nrptrs = min(DIV_ROUND_UP(iov_iter_count(i), PAGE_SIZE), > - PAGE_SIZE / (sizeof(struct page *))); > - nrptrs = min(nrptrs, current->nr_dirtied_pause - current->nr_dirtied); > - nrptrs = max(nrptrs, 8); > + nrptrs = get_nr_pages(fs_info, pos, i); > pages = kmalloc_array(nrptrs, sizeof(struct page *), GFP_KERNEL); > if (!pages) > return -ENOMEM; >