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From: Eric Dumazet <dada1@cosmosbay.com>
To: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Cc: Ulrich Drepper <drepper@gmail.com>,
	lkml <linux-kernel@vger.kernel.org>,
	David Miller <davem@davemloft.net>,
	Ulrich Drepper <drepper@redhat.com>,
	Andrew Morton <akpm@osdl.org>, netdev <netdev@vger.kernel.org>,
	Zach Brown <zach.brown@oracle.com>,
	Christoph Hellwig <hch@infradead.org>,
	Chase Venters <chase.venters@clientec.com>,
	Johann Borck <johann.borck@densedata.com>
Subject: Re: [take19 1/4] kevent: Core files.
Date: Thu, 5 Oct 2006 12:45:03 +0200	[thread overview]
Message-ID: <200610051245.03880.dada1@cosmosbay.com> (raw)
In-Reply-To: <20061005102106.GE1015@2ka.mipt.ru>

On Thursday 05 October 2006 12:21, Evgeniy Polyakov wrote:
> On Thu, Oct 05, 2006 at 11:56:24AM +0200, Eric Dumazet (dada1@cosmosbay.com) 
> > I may be wrong, but what is currently missing for me is :
> >
> > - No hardcoded limit on the max number of events. (A process that can
> > open XXX.XXX files should be allowed to open a kevent queue with at least
> > XXX.XXX events). Right now thats not clear what happens IF the current
> > limit is reached.
>
> This forces to overflows in fixed sized memory mapped buffer.
> If we remove memory mapped buffer or will allow to have overflows (and
> thus skipped entries) keven can easily scale to that limits (tested with
> xx.xxx events though).

What is missing or not obvious is : If events are skipped because of 
overflows, What happens ? Connections stuck forever ? Hope that everything 
will restore itself ? Is kernel able to SIGNAL this problem to user land ?


>
> > - In order to avoid touching the whole ring buffer, it might be good to
> > be able to reset the indexes to the beginning when ring buffer is empty.
> > (So if the user land is responsive enough to consume events, only first
> > pages of the mapping would be used : that saves L1/L2 cpu caches)
>
> And what happens when there are 3 empty at the beginning and \we need to
> put there 4 ready events?

Re-read what I said :  when ring buffer is empty.

When ring buffer is empty, kernel can reset index right before adding XX new 
events. You read 3 events consumed, I said : When all ring buffer is empty, 
because all previous events were consumed by user land, then we can reset 
indexes to 0.

>
> > A plus would be
> >
> > - A working/usable mmap ring buffer implementation, but I think its not
> > mandatory. System calls are not that expensive, especially if you can
> > batch XX events per syscall (like epoll). Nice thing with a ring buffer
> > is that we touch less cache lines than say epoll that have lot of linked
> > structures.
> >
> > About mmap, I think you might want a hybrid thing :
> >
> > One writable page where userland can write its index, (and hold one or
> > more futex shared by kernel) (with appropriate thread locking in case
> > multiple threads want to dequeue events). In fast path, no syscalls are
> > needed to maintain this user index.
> >
> > XXX readonly pages (for user, but r/w for kernel), where kernel write its
> > own index, and events of course.
>
> The problem is in that xxx pages - how many can we eat per kevent
> descriptor? It is pinned memory and thus it is possible to have a DoS.
> If xxx above is not enough to store all events, we will have
> yet-another-broken behaviour like rt-signal queue overflow.
>

Re-read : I have a process that has the right to open XXX.XXX handles, 
allocating XXX.XXX tcp sockets, dentries, files structures, inodes, epoll 
events, its obviously already a DOS risk, but controled by 'ulimit -n'

Allocating XXX.XXX * (32 or 64) bytes is a win if I can zap epoll structures 
(currently more than 256 bytes per event)

epoll structures are pinned too... what's wrong with that ?

# egrep "filp|poll|TCP|dentries|sock_inode" /proc/slabinfo |cut -c1-50
tw_sock_TCP         1302   2200    192   20    1 :
request_sock_TCP    2046   4260    128   30    1 :
TCP               151509 196910   1472    5    2 :
eventpoll_pwq     146718 199439     72   53    1 :
eventpoll_epi     146718 199360    192   20    1 :
sock_inode_cache  149182 197940    640    6    1 :
filp              149537 202515    256   15    1 :

If you want to protect from DOS, just use ulimit -n 100

Eric

  reply	other threads:[~2006-10-05 10:45 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <115a6230591036@2ka.mipt.ru>
2006-09-12  8:41 ` [take18 0/4] kevent: Generic event handling mechanism Evgeniy Polyakov
2006-09-12  8:41   ` [take18 1/4] kevent: Core files Evgeniy Polyakov
2006-09-12  8:41     ` [take18 2/4] kevent: poll/select() notifications Evgeniy Polyakov
2006-09-12  8:41       ` [take18 3/4] kevent: Socket notifications Evgeniy Polyakov
2006-09-12  8:41         ` [take18 4/4] kevent: Timer notifications Evgeniy Polyakov
2006-09-20  9:35 ` [take19 0/4] kevent: Generic event handling mechanism Evgeniy Polyakov
2006-09-20  9:35   ` [take19 1/4] kevent: Core files Evgeniy Polyakov
2006-09-20  9:35     ` [take19 2/4] kevent: poll/select() notifications Evgeniy Polyakov
2006-09-20  9:35       ` [take19 3/4] kevent: Socket notifications Evgeniy Polyakov
2006-09-20  9:35         ` [take19 4/4] kevent: Timer notifications Evgeniy Polyakov
2006-10-04  6:34     ` [take19 1/4] kevent: Core files Ulrich Drepper
2006-10-04  6:48       ` Evgeniy Polyakov
2006-10-04 17:57         ` Ulrich Drepper
2006-10-05  8:57           ` Evgeniy Polyakov
2006-10-05  9:56             ` Eric Dumazet
2006-10-05 10:21               ` Evgeniy Polyakov
2006-10-05 10:45                 ` Eric Dumazet [this message]
2006-10-05 10:55                   ` Evgeniy Polyakov
2006-10-05 12:09                     ` Eric Dumazet
2006-10-05 12:37                       ` Evgeniy Polyakov
2006-10-15 23:22                         ` Ulrich Drepper
2006-10-16  7:33                           ` Evgeniy Polyakov
2006-10-16 10:16                             ` Ulrich Drepper
2006-10-16 11:23                               ` Evgeniy Polyakov
2006-10-17  5:10                           ` Johann Borck
2006-10-17  5:59                             ` Chase Venters
2006-10-17 10:42                               ` Evgeniy Polyakov
2006-10-17 13:12                                 ` Chase Venters
2006-10-17 13:35                                   ` Evgeniy Polyakov
2006-10-17 10:39                             ` Evgeniy Polyakov
2006-10-17 13:19                               ` Eric Dumazet
2006-10-17 13:42                                 ` Evgeniy Polyakov
2006-10-17 13:52                                   ` Eric Dumazet
2006-10-17 14:07                                     ` Evgeniy Polyakov
2006-10-17 14:25                                       ` Eric Dumazet
2006-10-17 15:09                                         ` Evgeniy Polyakov
2006-10-17 15:32                                           ` Eric Dumazet
2006-10-17 16:01                                             ` Evgeniy Polyakov
2006-10-17 16:26                                               ` Eric Dumazet
2006-10-17 16:35                                                 ` Evgeniy Polyakov
2006-10-17 16:45                                                   ` Eric Dumazet
2006-10-18  4:10                                                     ` Evgeniy Polyakov
2006-10-18  4:45                                                       ` Eric Dumazet
2006-10-17 15:33                                         ` Hans Henrik Happe
2006-10-05 14:01                 ` Hans Henrik Happe
2006-10-05 14:15                   ` Evgeniy Polyakov
2006-10-05 15:07                     ` Hans Henrik Happe
2006-09-22 19:22   ` [take19 0/4] kevent: Generic event handling mechanism Andrew Morton
2006-09-23  4:23     ` Evgeniy Polyakov
2006-10-04  6:09       ` Ulrich Drepper
2006-10-04  6:10         ` Ulrich Drepper
2006-10-04  6:27           ` Evgeniy Polyakov
2006-10-04  6:24         ` Evgeniy Polyakov
2006-09-26 15:54     ` Christoph Hellwig
2006-09-27  4:46       ` Evgeniy Polyakov
2006-09-27 15:09   ` Evgeniy Polyakov
2006-10-04  4:50     ` Ulrich Drepper
2006-10-04  4:55       ` Evgeniy Polyakov
2006-10-04  7:33         ` Ulrich Drepper
2006-10-04  7:48           ` Evgeniy Polyakov
2006-10-04 17:20             ` Ulrich Drepper
2006-10-05  9:02               ` Evgeniy Polyakov
2006-10-05 14:45                 ` Ulrich Drepper
2006-10-06  8:36                   ` Evgeniy Polyakov
2006-10-15 22:43                     ` Ulrich Drepper
2006-10-16  7:23                       ` Evgeniy Polyakov
2006-10-16  9:59                         ` Ulrich Drepper
2006-10-16 10:38                           ` Evgeniy Polyakov

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