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[174.88.77.103]) by smtp.gmail.com with ESMTPSA id k64sm1676484qkc.97.2020.10.07.10.05.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 07 Oct 2020 10:05:42 -0700 (PDT) Sender: Mark Johnston Date: Wed, 7 Oct 2020 13:05:40 -0400 From: Mark Johnston To: SHAMANTHA KRISHNA K G Cc: freebsd-hackers@freebsd.org Subject: Re: swap vm object Message-ID: <20201007170540.GB92185@raichu> References: <20201007122452.GA92185@raichu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 4C611h4VLsz3Y3C X-Spamd-Bar: -- Authentication-Results: mx1.freebsd.org; dkim=pass header.d=gmail.com header.s=20161025 header.b=eTomSlZ9; dmarc=none; spf=pass (mx1.freebsd.org: domain of markjdb@gmail.com designates 2607:f8b0:4864:20::842 as permitted sender) smtp.mailfrom=markjdb@gmail.com X-Spamd-Result: default: False [-2.46 / 15.00]; RCVD_VIA_SMTP_AUTH(0.00)[]; TO_DN_SOME(0.00)[]; R_SPF_ALLOW(-0.20)[+ip6:2607:f8b0:4000::/36:c]; RCVD_COUNT_THREE(0.00)[3]; DKIM_TRACE(0.00)[gmail.com:+]; RCPT_COUNT_TWO(0.00)[2]; NEURAL_HAM_SHORT(-0.71)[-0.709]; FREEMAIL_TO(0.00)[gmail.com]; FORGED_SENDER(0.30)[markj@freebsd.org,markjdb@gmail.com]; MIME_TRACE(0.00)[0:+]; FREEMAIL_ENVFROM(0.00)[gmail.com]; ASN(0.00)[asn:15169, ipnet:2607:f8b0::/32, country:US]; FROM_NEQ_ENVFROM(0.00)[markj@freebsd.org,markjdb@gmail.com]; ARC_NA(0.00)[]; NEURAL_HAM_MEDIUM(-1.02)[-1.019]; R_DKIM_ALLOW(-0.20)[gmail.com:s=20161025]; FROM_HAS_DN(0.00)[]; NEURAL_HAM_LONG(-1.03)[-1.030]; MIME_GOOD(-0.10)[text/plain]; PREVIOUSLY_DELIVERED(0.00)[freebsd-hackers@freebsd.org]; DMARC_NA(0.00)[freebsd.org]; TO_MATCH_ENVRCPT_SOME(0.00)[]; RCVD_IN_DNSWL_NONE(0.00)[2607:f8b0:4864:20::842:from]; MID_RHS_NOT_FQDN(0.50)[]; RCVD_TLS_ALL(0.00)[]; MAILMAN_DEST(0.00)[freebsd-hackers] X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.33 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 07 Oct 2020 17:05:45 -0000 On Wed, Oct 07, 2020 at 10:19:12PM +0530, SHAMANTHA KRISHNA K G wrote: > Hello Mark, > Thank you very much for the heads up,may you please tell how it > differs from a default vm object. An OBJT_DEFAULT VM object is just a swap object for which the swap pager holds no blocks. The first time that the system pages out from a default object, it gets converted to a proper swap object; see the beginning of swap_pager_putpages(). Default objects only exist as an optimization: certain operations on default objects are cheaper because the kernel knows it can avoid interrogating the swap pager. In many cases default objects are short-lived and never undergo a pageout operation. This optimization may be less important now than it used to be: r322913 replaced a global object+pindex->swap block hash table with per-object trees mapping page indices to swap blocks. > On Wed, 7 Oct 2020, 19:00 Mark Johnston, wrote: > > > On Wed, Oct 07, 2020 at 01:28:34PM +0530, SHAMANTHA KRISHNA K G wrote: > > > Hello All , > > > > > > What is a swap vm object in case of /proc//map ? > > > > It represents memory that is backed by the swap device. If the system > > is forced to reclaim memory from this object, it will first use the swap > > pager to write the pages' contents to a swap device. Then, a subsequent > > access can recover the data by paging in from the swap device. > > > > Often such objects contain anonymous pages, such as those allocated > > using malloc(). They are also used for certain persistent objects, such > > as tmpfs or shared memory files. > >