in reply to Re^3: 4k read buffer is too small
in thread 4k read buffer is too small

I'm convinced that modifying perl's block size would be a wasted effort. It would not change the size of the NFS requests to the server.

I disagree with your judgement. Please refer to the mtab string I posted further down. The rsize mount option is 32k, and was all along.

read and rsize match:
If the read buffer matches the NFS rsize mount option, then each read will incur 1 NFS request to the server with maximum possible efficiency.
read less than rsize:
A read with a buffer smaller than the rsize option cannot wait until the NFS buffer is filled - the NFS client will issue a request with however little was requested, unless the data is already in the read-ahead buffer. Again, each read will incur 1 NFS request, albeit less efficient by increasing the header to data ratio (more overhead).
read larger than rsize:
A read buffer larger than the rsize option will be split by your friendly VFS layer into however many NFS requests it takes to fulfill using rsize-sized chunks.

In my concrete example, this means that the 4k buffer employed by PerlIO will issue more NFS request than using a 32k buffer that matches my rsize mount option. Thus, increasing the buffer size in PerlIO is not wasted effort. Rather, any increase to 32k or beyond would result in a factor 8 less NFS requests. AFAI understood my admins, it is the sheer number of small NFS requests that slows the server to a crawl.

Careful testing will reveal the ideal buffer size. But both, the admins and I expect it to be at 32k or larger, not at 4k.

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Re^5: 4k read buffer is too small
by starbolin (Hermit) on Jun 18, 2008 at 18:06 UTC

    From your quoted text:

    "...unless the data is already in the read-ahead buffer."
    So what's happening here is you perl script is faster that the NFS fill ( not suprising ) and so is emptying out the buffer on each read. Subsequent reads are issued before the read-ahead has time to fill the buffer. At least in a perfect world this what 'may' be happening, unless your NFS client is broken somehow. Try putting a sleep in your script and look at iostat to see if the NFS read sizes increases. I had actually though earlier of putting sleep in the code to ease the burden on the network but dismissed it as not addressing the NFS driver buffering issue, but from the text provided it seems that your driver implementation over-responds to multiple requests and so might respond to a slowing of the request rate.

    I don't recall if you've stated anywhere whether your script reads the file sequentially. If so have you tried somethine like: 

    cat "filename" > script

    [download]
    In order to buffer the reads. ( You may need an enhanced version of cat to avoid memory thrashing with this method.)

    Should you still decide to pursue modification of perl it would seem that this is the kind of thing for which IO layers was implemented. One would make a copy of the appropriate layer, change the name, then modify the buffer size. Switching to a larger read buffer would then simply require a use statement in your code.

    s//----->\t/;$~="JAPH";s//\r<$~~/;{s|~$~-|-~$~|||s |-$~~|$~~-|||s,<$~~,<~$~,,s,~$~>,$~~>,, $|=1,select$,,$,,$,,1e-1;print;redo}
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