Double Scan 95 "RAM Doubler" Performance/Compression Analyzer

Introduction

The Double Scan utiltity determines the fraction of a program's data which is compressed by "RAM doublers", as a tool to validate and compare manufacturers' claims.

[Sample output from DblScan]
Sample output from DblScan, while running SoftRAM 95 under Windows 3.1, with a permanent swap file. Note that "Total uncompressed pages found" is equal to "Pages Allocated." In contrast, here is DblScan output, also from Windows 3.1, but with a genuine RAM compressor, RAM Doubler from Connectix, installed:

Notice that the number of uncompressed pages found by DblScan is less than the number of pages allocated: some of the pages were compressed by RAM Doubler. Similar (actually, in this case, even better) results are achieved with MagnaRAM from Quarterdeck, another genuine RAM compressor. Now note the results from DblScan, with no RAM-compression software at all:

Of course, the number of uncompressed pages is identical to the number of pages allocated, just as if SoftRAM were installed. Notice too that more time is used (25 sec.) than when a genuine RAM compressor is installed (14 sec.), but less time than when SoftRAM is installed (27 sec.).

Installing Double Scan

Double Scan runs under Windows 3.1 and Windows 95.
To install Double Scan, unpack the files into a directory such as c:\dblscan. Next, add a line to "[386Enh]" section of your "\windows\system.ini" file of the form:
device=c:\dblscan\dblscan.386

Running Double Scan

Begin Double Scan as you would any other Windows program. The Help button brings up this readme file. The Start button starts the test. After hitting Start, you will be prompted for a delay period and a scan string. The delay period represents the length of time that Double Scan will wait after its test before it scans memory to accumulate results. This delay gives RAM doublers that claim to compress during Windows idle periods the opportunity to do so.
The scan string represents the text information that Double Scan fills the memory it allocates with. It is not really important what the string is as any reasonable compression algorithm will have no problem compressing repeated strings of 8 characters in length, but the compressability will be increased further if you choose a string of 8 identical characters.
Double Scan requires several seconds to several minutes to perform its analysis, during which time your machine may not be useable. To ensure accurate and reproducible results you should reboot the machine prior to each run, and ensure that no other programs are currently running. Running the program twice without rebooting may confuse Double Scan into miscounting the number of pages being compressed.
See the "Interpreting results" section below for help on how to interpret the output of the program.

Background

The size of a computer's RAM memory determines the amount of information that the system can directly access. Each program, or block of data used by a program, requires a portion of the valuable RAM space proportional to the program's size. When a machine's RAM is exhausted the system will either fail to load additional programs or data, or will be forced to begin storing some of the information on the hard disk instead of RAM. Data being stored on the hard disk cannot be directly accessed by the machine; it must first be copied into RAM, usually displacing other information which is then copied to the hard disk. The process of copying data from disk before accessing it can be thousands of times slower than accessing directly in RAM memory. (This behavior is refered to as "paging" or "virtual memory".)

RAM "multipliers"

The principal behind "RAM doublers" is that by compressing the information in a computer's RAM memory, more programs and data can reside in the machine simultaneously, and the need to copy information to and from the hard disk is reduced. The result is that more and bigger programs can execute simultaneously, and performance is increased because more information can be accessed directly. (Performance will never be as good as if real additional RAM chips are added to the machine, because the overhead of compressing and uncompressing the data slows the machine).

Double Scan behavior

The Double Scan utility verifies the functionality of a RAM multiplier (the term "doubler" is a misnomer because it is impossible to guarantee the specific amount by which data can be compressed and thus memory increased). It detects what fraction of the data a program uses is actually being compressed, and the proportion that is stored in RAM versus stored on the hard disk. Double Scan accomplishes this by allocating and using an amount of memory larger than the system's RAM memory. In order to satisfy this request the system is forced to either store some of the information on the hard disk, or if equiped with a RAM multiplier, to compress some of the information in RAM. While running, Double Scan audits the information being written to disk, and searches the RAM memory to determine if it can locate its data in an uncompressed form. It then reports the total counts to the user.

Interpreting results

Double Scan reports its results in terms of memory "pages." A page of memory is 4096 bytes (4 kilobytes), and when the system allocates or manipulates RAM memory it does so in chunks of this size. Similarly, when RAM memory must be copied to or from the hard disk it is done in blocks of this size.
The meanings of the numbers reported by Double Scan are as follows:
Pager type: This entry tells you what type of mechanism is being used by the pager to write to the disk. Here are the possible settings:

    Win3.1      Paging Type
    ------      -----------
    NOPG        No paging activity was encountered hence the
                type of pager was not detected
    DOS         You have a temporary swap file so DOS is being
                used to access it
    BIOS        You have a permanent swap file, but your 32-bit
                disk access is disabled
    BLCK        You have a permanent swap file and 32-bit disk
                access is enabled

    Win95       Paging Type
    -----       -----------
    NOPG        No paging activity was encountered hence the
                type of pager was not detected
    DOS         Windows does not recognize your hard drive's type
                so its using DOS to do the accesses
    IFS         The normal case where Windows is using the IFS
                manager to access the swap file

Number of disk writes: The number of times the system copied information from memory onto the hard disk. A larger number translates to worse performance. If compression is taking place this number should be smaller than without compression, since a greater fraction of information will be kept in RAM rather than on the disk.
Number of pages allocated: This is the total amount of memory that Double Scan requested from the system. Some fraction of this memory will be in RAM, some will be on the hard disk, and some will be compressed by any existing RAM multiplier.
Number of pages in RAM: This is the number of pages that Double Scan allocated that were stored in RAM in an uncompressed form.
Number of pages on disk: This is the number of pages that Double Scan allocated that were stored on the hard disk in an uncompressed form.
Total pages found: This is the number of pages that Double Scan allocated that were uncompressed by a RAM multiplier. If this number comes out to a number larger than the total pages allocated, it means that there is some internal buffering being used by either the RAM doubler or the disk subsystem and it represents redundant un-used information.
Total time: This is the amount of time required to run the experiment not including clean-up time at the end. A quality RAM multiplier should reduce this time by reducing the amount of information copied to the disk. If you specified a delay for memory scan, then the time represents time up to the point where Double Scan started to wait for the delay period to expire.
If the number of pages reported as being compressed is close to zero, then the RAM multiplier you have installed is not functioning correctly.
If a good RAM doubler is being used, then what you should see is the following:

a drop in the number of disk writes under the RAM doubler versus without, since more pages are kept in RAM due to compression
a drop in the total time under the RAM doubler since there are fewer disk accesses and compression should be faster than disk accesses
a drop in the number of pages found on disk and in RAM since compression will prevent Double Scan from finding its signature

IF YOU DO NOT SEE ANY DIFFERENCE IN PAGES FOUND AND PAGES ALLOCATED THERE IS NO COMPRESSION BEING USED

Example Results

A thorough comparison has been made between native Windows, Syncronous Softcorps.'s SoftRAM95, Quarterdeck's MagnaRAM2 and Connectix's RAM Doubler (Windows 3.1 only). Runs were executed for each setup for each of the three Windows 3.1 paging types (see above) as well as Windows 95.
Example screen-shots for runs under Windows 3.1 are in the Windows 3.1 Write file "rslt31.doc." Example screen-shots for runs under Windows 95 are in the WordPad document "rslt95.doc."
The following table summarizes the results of the experiments:

Windows 3.1: Temporary Paging File, DOS Paging
----------------------------------------------
        Pages Compressed        Time
Native            0              31 
SoftRAM           0              34
RAM Doubler     818              14     
MagnaRAM       1617              15

Windows 3.1: Permanent Paging File, BIOS Paging
-----------------------------------------------
        Pages Compressed        Time
Native            0              25
SoftRAM           0              27
RAM Doubler     918              14     
MagnaRAM       1565              12

Windows 3.1: Permanent Paging File, Blockdev Paging
---------------------------------------------------
        Pages Compressed        Time
Native            0              21
SoftRAM           0              19
RAM Doubler     918              14     
MagnaRAM       1344              12

Windows 95: IFS Manager Paging
------------------------------
        Pages Compressed        Time
Native            0              18
SoftRAM           0              18
RAM Doubler     N/A             N/A
MagnaRAM       3040              12

The results indicate that under the test conditions, SoftRAM does not perform any compression while MagnaRAM and RAM Doubler both do. Runs were made on SoftRAM with delays of 30 seconds and the results were identical to those in the above summary table. This result is consistent with the conclusion that SoftRAM does not use compression.

The Double Scan algorithm

Double Scan detects the number of pages being compressed by allocating pages and filling them with an easily recognized signature string. It hooks the disk read/write services, and while executing counts the number of pages that are written to disk containing this signature. Written pages that have been compressed will no longer have the signature clearly visible, while uncompressed pages are easily detected.
When all memory is allocated and filled (some of which will have been written to disk), Double Scan scans through the machine's RAM and counts the number of pages in RAM that have not been compressed. The total number of uncompressed pages is computed by adding the uncompressed count on disk and uncompressed count in RAM.

Modifying and recompiling the sources

Double Scan is free software, and may be freely modified and redistributed. Double Scan was built using Visual C 1.5 and VTools 1.11.

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